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]

I'm gonna try to reduce the amount of migration changes involving views, while keeping the ability to revert all migrations without errors being thrown

[dessalines]

Sry about being slow to review this, I'd like it to come after the 0.19.4 release.

[dessalines]

The editing (even older broken migrations) seems a bit scary (but you did explain why you need to do this.). And I don't understand the reason for all of the custom code for diff checking.

But overall I think this is a good idea.

[dessalines]

We can't check / test the migrations anymore in CI?

[dessalines]

Why is so much custom code necessary for checking a diff?

[dessalines]

These are really old migrations that are updating tables and views that aren't used anymore, whats going on?

[dullbananas]

I've wanted to split this PR so the changed diff behavior and the migration changes it requires are separate, but I've been putting it off. I will do that soon.

For the migration changes, I now think the diff checker should only check migrations after the migration that removes all views. That should allow removing most of the migration changes. I will work on this for a while after I make the PR with the simpler changes.