humbug: Add checkbox for emphasizing branch in topic.

[timabbott]

I realize ordinarily you folks are not accepting feature changes; but for complicated reasons it'd be quite valuable for us to be able to get this one last feature change in (so we can stop hardcoding this for particular users on the backend) in a timely fashion. I expect we'll be able to switch to the webhook based model before this needs to be modified again. Thanks for your consideration!

[kdaigle]

@timabbott I'm really sorry but you're right when you've pointed out that we won't be accepting any feature changes to existing services. I'm going to have to close this PR.

If necessary, consider overloading one of your existing fields to handle the change for the particular, small set of users. For example, make your branches config field have a + at the end of each branch name if they are to be emphasized. We could accept documentation updates but no feature updates.