Fixes and tests for metadata labels

[rtibbles]

Summary

• Adds extensive testing for searching by metadata labels and the available labels generation code
• This showed that the available metadata labels code was not quite doing what it ought
• Changes how the available metadata labels code produces the available metadata labels to make the tests pass and also improve performance
• Uses a Bit wise OR in Postgres to determine which labels are present
• Implements a Bit wise OR shim for SQLite that functions identically to the postgres Bit wise OR, except that the maximum size of bits must be specified in advance
• Uses a bit wise OR to do the label aggregation in a single query without subqueries, which significantly improves the performance, and gives full test compliance
• Limits the data returned for Channels and Languages in the available label metadata to just the ids (reduces query times for these two specific items by roughly 100x for each)
• Bootstraps full language data into the learn page explicitly to compensate for only the ids (to compensate for the full data no longer coming from the available labels)

Fixes #8737

Reviewer guidance

Does filtering seem to produce the correct remaining labels?
When testing I changed this line to if True: https://github.com/learningequality/kolibri/blob/release-v0.15.x/kolibri/core/content/utils/search.py#L91 to repeatably execute the queries that are being optimized here.

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Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

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Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[pcenov]

Tested in this build and the 500 error is fixed now plus filtering is fully functional.

[bjester]

Couple comments, but looks good!

[jredrejo]

I have tested it as much as I have been able (in sqlite only) and it has worked perfectly.
Also, seeing how pytest kolibri/core/content/test/test_search.py results in
371 passed in 63.65 seconds it's quite impressive.
Great work!