feat: add priority option

[Sakib25800]

This PR adds a priority option to @bors r=<username> <p=priority> and @bors r+ <p=priority>.

[Sakib25800]

Prerequisite to #202

[Kobzol]

Looks pretty good! Left some comments. And there should also be some tests (e.g. run @bors r+ p=... twice in a row and similar cases).

Then we'll add more test cases once @bors p=5 is implemented.

[Sakib25800]

Thanks! I'll make those changes and @ you when ready.

[Sakib25800]

@Kobzol do we need a max_priority option like Homu has?

[Kobzol]

@Kobzol do we need a max_priority option like Homu has?

I don't think so, I haven't ever seen it being useful/used anywhere.

[Sakib25800]

If there's any further changes needed, lmk.

[Kobzol]

Please also add two following tests:

• User sets priority with @bors p=X, and then does @bors r+, and check that the priority is still X.
• User sets priority with @bors p=X, and then does @bors r+ p=Y, and check that the priority is now Y.

Then I think we're done here :)

[Sakib25800]

Added priority_preserved_after_approve and priority_overridden_on_approve_with_priority 🫡.

[Kobzol]

Thank you! This is really impressive work.