"Update branch" button for unprotected branches

[earonesty]

Feature request (ccing support):

Add an "Update Branch" button for all branches that are out of date - even without branch protection.

Otherwise we have to either:

a) require all users to always manually merge them from the command line when needed. or

b) require our master branch reqs to always be up to date... even when it's not necessary

[cirosantilli]

Related: #121

[jd]

@earonesty Some of this might be automatically be done with https://mergify.io AFAIU. It can merge branches automatically when needed before merge.