Add PR check to require an assigned milestone prior to merging

[westonruter]

Feature description

One of the suggestions in #2741 is:

Automatically assign the correct milestone to newly opened pull requests

This would not work so well for us because we'll have the next major milestone (x.y) along with the next minor milestone (x.y.z) that we need to decide between. Therefore, the most important thing is that a milestone needs to be specified. So a new check could be introduced that blocks merging of a PR until a milestone is assigned.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[thelovekesh]

@westonruter Do we still need this?

[westonruter]

It's not important, nah.