Create subject counts on collection

[yuenmichelle1]

Create a counter_cache column (subjects_count) in order to filter out collections by number of subjects.

TODO after merge, deploy, migration

• Follow up PR to filter collections by min_subjects (PR: WIP - Filter collections by min num of subjects #4634)
• Run Rake task to backfill collections subjects_count

Review checklist

• First, the most important one: is this PR small enough that you can actually review it? Feel free to just reject a branch if the changes are hard to review due to the length of the diff.
• If there are any migrations, will they the previous version of the app work correctly after they've been run (e.g. the don't remove columns still known about by ActiveRecord).
• If anything changed with regards to the public API, are those changes also documented in the apiary.apib file?
• Are all the changes covered by tests? Think about any possible edge cases that might be left untested.