Use pg_repack to remove bloat from postgres
There's roughly 500 GB of bloat in the database (in addition to about 1.4 TB of actual data/indexes). See gitlab-com/database#110 (comment 97473439).
While merge_request_diff_files (with 156 GB of bloat) is the most interesting table for this, we have a lot of bloat all over the place. This issue is about deploying pg_repack to address the bloat issue.