Add a label to mark which ANV round the proposal is for

[Smokyish]

Currently PRs have labels for different stages of going through the process.

As there will be past proposals and upcoming that want to get included in a specific round, I think it would be useful to have Round/Vote labels added as well.

So that for example, AGP-5 would have a label for Aragon Network Vote #1 where as AGP-42 would have a Aragon Network Vote #2 label attached.

[john-light]

I don't think this is necessary, and as proposals can be proposed across multiple votes (e.g. AGP-9) it is just one more thing to update. Any PR that is open between the last vote and the next vote is by default under consideration for the next vote.

If people are interested in which vote an AGP was or will be part of, then we do have a directory with a historical record of past/upcoming votes:

https://github.com/aragon/AGPs/blob/master/votes