R1:0a77b0e53e7f

Work around an issue in MariaDB where dropping a column from a UNIQUE KEY fails

Summary:
See T13193. See T13077. If we drop a column which is part of a UNIQUE KEY, MariaDB raises an error.

This is probably a bad idea on our side anyway, but in this case it wasn't an obviously bad idea.

To get around this:

- Drop the unique key, if it exists, before dropping the column.
- Explicitly add the new unique key afterward.

Test Plan: Ran `bin/storage upgrade` locally without issue, but I'm on MySQL. Will…