Make branch input boxes into dropdowns (or select elements)

[jseppi]

It seems like we already have access to the list of branches in a repository (since they are listed in the "GitHub Branches" table). It would be a nicer user experience, I think, to have a dropdown box or <select> element for all the places where the user needs to specify a branch name instead of having to type it.

[wslack]

Add that for adding a new site

[jseppi]

After #1027 is done, users will not be presented with a table of GitHub branches on the Site Settings page. That will maybe make it a little harder for them to fill out the branch and demoBranch text fields because they won't have a list of branches right in front of them. We might want to increase prioritization of this issue to make branch selection more user friendly.

[wslack]

I'm ok with this not being prioritized right now (unless it takes only an hour of work) given how rarely these settings are changed.

[wslack]

@el-mapache another possibility

[apburnes]

Closing for new round of UX research.