Fix cache pruning

[rogermparent]

Fixes #1405, which I've confirmed is a cache issue with local testing.

We totally have a tool that prunes cache to solve that, I must have accidentally broken the invocation earlier before starting work on the CML site.

This PR turns the weird function-but-not-model pruner into a model like the other pieces of gatsby-node logic we use, both cleaning it up and fixing the invocation.

[rogermparent]

From the preview logs:

       Removed stale page data at "public/page-data/doc/command-reference/metrics/add/page-data.json"

       Removed stale page data at "public/page-data/doc/command-reference/metrics/modify/page-data.json"

       Removed stale page data at "public/page-data/doc/command-reference/metrics/remove/page-data.json"

       Removed stale page data at "public/page-data/doc/command-reference/lock/page-data.json"

       Removed stale page data at "public/page-data/doc/command-reference/unlock/page-data.json"

       Removed stale page data at "public/page-data/doc/command-reference/pipeline/show/page-data.json"

       Removed stale page data at "public/page-data/doc/command-reference/pipeline/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/deep/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/community/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/deep/reproducibility/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/deep/define-ml-pipeline/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/add-files/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/deep/preparation/page-data.json"

       Removed stale page data at "public/page-data/doc/command-reference/pipeline/list/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/agenda/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/connect-code-and-data/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/data-access/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/data-pipelines/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/data-versioning/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/experiments/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/import-data/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/initialize/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/metrics/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/older-versions/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/pipeline/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/reproduce/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/retrieve-data/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/store-data/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/visualize/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/interactive/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/pipelines/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/versioning/page-data.json"

       Removed stale page data at "public/page-data/doc/understanding-dvc/collaboration-issues/page-data.json"

       Removed stale page data at "public/page-data/doc/understanding-dvc/core-features/page-data.json"

       Removed stale page data at "public/page-data/doc/understanding-dvc/existing-tools/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/deep/sharing-data/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/compare-experiments/page-data.json"

       Removed stale page data at "public/page-data/doc/tutorials/get-started/configure/page-data.json"

       Removed stale page data at "public/page-data/doc/understanding-dvc/how-it-works/page-data.json"

       Removed stale page data at "public/page-data/doc/understanding-dvc/related-technologies/page-data.json"

       Removed stale page data at "public/page-data/doc/understanding-dvc/resources/page-data.json"

       Removed stale page data at "public/page-data/doc/understanding-dvc/what-is-dvc/page-data.json"

       Removed stale page data at "public/page-data/doc/user-guide/dvc-file-format/page-data.json"

Wow, that's more than I thought there would be! Anyway, this confirms the pruner working in this PR.

[rogermparent]

https://dvc-landing-fix-cache-p-yjhtrb.herokuapp.com/doc/command-reference/metrics/add

And here's a proper 404 on a previously stale page!

[shcheklein]

Thanks @rogermparent !