Strawman policy on assigning tickets in github.

We seem to have two strategies for managing who's working on which ticket:

1. Assign tickets to the person who is going to work on it and then use the `now` and `next` labels to manage.
2. Only assign to yourself tickets when you have started, or are the only person who should do, the ticket.

For projects with several people on 2 seems better so it is what I've proposed. I find the overhead of managing the name and next labels tedious so don't like 1.