Add rake task to clean cache

[damianhxy]

Description

• Expire cache contents after 7 days (with race_condition_ttl set to 1 minute -- used to avoid race conditions)
• Add a rake task to facilitate cleaning the cache
• Update troubleshooting portion of documentation

Motivation and Context

On 27 Oct, Autolab started returning 500 internal server errors due to the cache disk running out of inodes when attempting to write to the raw score caches. (autolab-error.pdf)

This is because FileStore caches (the default cache) are not purged automatically.

Reference: https://stackoverflow.com/questions/27502082/clearing-expired-cache-entries-from-disk-cache-on-ruby-on-rails-4

Unfortunately, there does not appear to be an easy way to periodically clear expired entries. In lieu of that, this PR adds a rake task and updates the documentation to facilitate the process

How Has This Been Tested?

Preliminary steps

• Enable caching in development by running rails dev:cache
• In development.rb, replace config.cache_store = :memory_store with config.cache_store = :file_store, "tmp/cache/scores"

Test that rake task clears expired entries

• For testing purposes, change :expires_in and race_condition_ttl to 5.seconds (in all 3 places)
• Load the gradesheet of an assessment (e.g. AutoPopulated / Homework0), observe that the path <AUTOLAB DIR>/tmp/cache/scores is populated
• Wait a few seconds, run rake user:cleanup_cache, observe that the (expired) entries are cleared

Test that rake task does not clear unexpired entries

• Change :expires_in and race_condition_ttl back to their original values
• Load the gradesheet of an assessment (e.g. AutoPopulated / Homework0), observe that the path <AUTOLAB DIR>/tmp/cache/scores is populated
• Wait a few seconds, run rake user:cleanup_cache, observe that the unexpired entries are not cleared
• As a sanity check, run rake user:clear_cache and observe that the entries are cleared

Documentation

• Run docs locally (mkdocs serve) and navigate to http://127.0.0.1:8000/installation/troubleshoot/#no-space-left-on-device-error

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• I have run rubocop for style check. If you haven't, run overcommit --install && overcommit --sign to use pre-commit hook for linting
• My change requires a change to the documentation, which is located at Autolab Docs
• I have updated the documentation accordingly, included in this PR

[20wildmanj]

Verified that cache cleanup task successfully works for filestore cache, and expiration time works, and checked docs. LGTM