Fix server error on user profile vote summary tab if there are orphaned votes

[Oaphi]

Closes #1488

This PR fixes the issue with orphan votes (with post_id but no actual post) by:

• making the vote_summary view resilient to entries with missing posts;
• making the UsersController#vote_summary filter out votes without associated posts;
• adding a CleanupVotes daily job that removes orphaned votes (those that do not correspond to any post but still have a post_id assigned to them);
• making Vote model more resilient to post not being present;

[cellio]

LGTM based on inspection (one paranoid request). I appreciate the extra resilience in the view and controller.

Someone with more knowledge of the scripts/daily jobs should also review this.

How do we test it? Can we inject some bad data on the dev server and test there before going to production?

[Oaphi]

How do we test it?

There are several ways of doing so. A simple prerequisite is voting on something, then intentionally breaking the database integrity by updating the record in the votes table to contain a non-existent post_id. If done right, you will see an entry in the target user's vote summary tab that says "Post not found".

After that's done, it's just a matter of executing the job. The simplest approach is to change the associated script file to call perform_now instead of perform_later and simply run it as a script via bin/rails runner <path_to_script> (in fact, this is basically what whenever does under the hood). There is no reason to test the schedule as it's managed by whenever (it sets up a cron job), although that can be done too