Add custom migration runner, forbid some diesel migration commands, fix old migrations

[dullbananas]

Reverting and testing migrations will be done with lemmy_server migration commands, and diesel migration commands will do nothing and fail when trying to run or revert migrations.

Benefits:

• Changes in replaceable_schema will no longer require a corresponding migration
• Migrations will now be run and reverted without the "r" schema applied, instead of needing to work both with and without it
• All migrations, not just the newest one, will be tested with schema diff
• Multiple lemmy_server processes trying to run migrations at the same time will now be prevented
• Name and duration of each migration will be displayed when run in lemmy_server

#4333 (comment)

The new tests (even just reverting all migrations) revealed some mistakes in old migrations, so I fixed those, and I also discovered that 'now'::timestamp in old views used the timestamp of the view's creation instead of the current timestamp.

[dullbananas]

The downgrade instructions will need to be changed to instead use the revert command (e.g. lemmy_server migration --number 3 revert to revert 3 migrations)