Add completeness counts to subject sets

[eatyourgreens]

For Engaging Crowds, we'd like to display subject set completeness when giving volunteers a choice of subject sets to work on for a particular workflow. It would be useful to have completeness, or number of completed subjects, recorded per subject set, if that's possible.

Here's a design for the project home page, showing how completeness will be listed for each subject set.

[eatyourgreens]

One possible complication here: a subject set can be linked to multiple workflows, and will have different completeness counts for each workflow that uses it.

[eatyourgreens]

There's also conversation on zooniverse/front-end-monorepo#1884, around how we'll want to list individual subjects from a set and show workflow status for each one, for the current user.

[camallen]

Linking zooniverse/front-end-monorepo#1884 (comment)

One naive approach could be to do this when doing the project completion counts on the hourly schedule? https://github.com/zooniverse/panoptes/blob/e4dcb068ad39bddd960423ef6e794f9917e940ca/app/workers/calculate_completeness_worker.rb

I'm not sure what kind of load the above would add to the system, it might be more prudent to modify the set counts on each retirement event?

panoptes/app/workers/retirement_worker.rb

Line 12 in e4dcb06

WorkflowRetiredCountWorker.perform_async(status.workflow_id)

Once those counts are on the SubjectSet resource we can serialize them via the API on the subject set resource.