Add PR/issue management things

[jsstevenson]

Require PR labels:

on:
  pull_request:
    types:
      - opened
      - labeled

jobs:
  check_priority_label:
    runs-on: ubuntu-latest
    steps:
      - name: "Check for required priority labels"
        if: ${{ !contains(github.event.pull_request.labels.*.name, 'priority:low') && 
                !contains(github.event.pull_request.labels.*.name, 'priority:medium') && 
                !contains(github.event.pull_request.labels.*.name, 'priority:high')}}
        run: exit 1

[korikuzma]

🫡

[korikuzma]

This was added in #11 (I forgot there was an issue for this already)