Implement GitHub actions to sync status labels

[soehms]

    > > Github sadly doesn't have a good search function for "has a review requiring changes" and thus many repos use a "needs review" and "needs work" label to indicate what is required to move this PR forward.

OK, good points there. I'll map them to labels. Preparation for this is in 9c0c175. Probably best to refactor label handling next.

It's a good idea to have needs_review or needs_work as labels. On the other hand, since this is redundant information, we need to sync these labels with the GitHub states. Otherwise they will immediately run out of reliability.

I think we can use triggers like this for that.

Originally posted by @soehms in #8 (comment)

[mkoeppe]

Similarly, when setting the label invalid, we could remove priority labels. For the imported tickets, I'm doing this in #127