Significantly increase scope of linter to catch type errors

[Cyclic3]

This PR is made up of a couple of pieces that can be cherry-picked if needed:

1. Replaced pylint with ruff in the GitHub Action for a much faster lint, added mypy as a linter, and parallelised the Aaction.
2. Added a corresponding ruff.toml, with enough of it's sillier lints turned off that the code passes through reasonably smoothly
3. Several small optimisations that come from this lint config
4. A bunch of MyPy annotation spam, as well as a small refactor of the ShareReminder class to get it to play ball
   This should hopefully make it a bit easier for bugs to be caught before they hit prod, without actually getting in the way of anybody's workflow.

I don't have admin access yet, so I'll keep this as a draft until there's been a chance to actually check that I haven't broken anything.

[Cyclic3]

To get the last part of mypy's type lints to pass, I need someone to confirm that the parse_attribute function should never return a private DM channel. I'll put through a commit with that assumption. But I need someone to check.