Verify integrity of uploaded files

This came out of discussion with @andrewn

Currently, we store a checksum in the uploads table for every file that is uploaded.

Bit rot or other problems (e.g., corruption during a Geo primary -> secondary file sync) could cause corruption of the file.

It would be nice to have a way to manually check the integrity of all files by verifying the copy on disc against the hash in the database. This could just be a rake task, or a more complicated "housekeeping" process fired off and monitored from the GitLab UI.

/cc @jramsay