Settings->Branches

2 new pages:
- How to prioritize a branch?
- How to skip building new branches?

Please review.

Generally we should avoid naming the pages "How to..." unless it's more confusing without, and definitely not end them with a question mark.

Ideas for these two pages:

- Set high priority branches
- Turn off automatic testing of new branches

Please also change the current "How to skip building some commits?" to "How to skip building some commits with [ci skip]".