Add scheduled job to clean up orphaned inbox entries

[pixeldesu]

Feature and bugfix in one PR!

This PR adds sidekiq-scheduler and our first scheduled job to go along with it, InboxCleanupScheduler.

This job runs every minute and cleans up orphaned inbox entries. This is a bug fix to prevent broken inboxes due to "somewhere down the line" deletion of something causing this behaviour. There is no case where this should happen, as to why the test case for this scheduler even has to disable validations on saving the broken record that is scheduled for deletion.

@nilsding this works fine locally without any change to the setup, but I'm not sure about how this ends up working with our deployment and what changes need to be made to accommodate this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a8cb2f6) 95.71% compared to head (6912cf7) 95.76%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1581      +/-   ##
==========================================
+ Coverage   95.71%   95.76%   +0.04%     
==========================================
  Files         174      175       +1     
  Lines        2706     2714       +8     
==========================================
+ Hits         2590     2599       +9     
+ Misses        116      115       -1     

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