Investigate limit on number of columns per table

From the discussion in #205207 (comment 286947758):

• Number of columns per table is limited to 1,600
• Dropped columns count towards that limit: https://asciinema.org/a/MJpVUPeGCxHfqQCorqwLL8CDo

I created this issue to verify this, investigate what can be done about it and check in what state GitLab.com is.

Summary:

• Dropped columns count towards the column limit as expected
• On GitLab.com, we still have lots of room here (see #205232 (comment 287156439))
• VACUUM FULL and pg_repack are not fixing the problem
• A full table re-create (with data copied between tables) fixes the problem
• Logical replication - perhaps we'll use it for the GitLab.com upgrade - resets the problem for GitLab.com