WIP render branch list picker

[edewit]

There are still 2 issues with this:

1. The branch list endpoint seems to return [] in all cases
2. Do we need to call the "buildImage suggest" again after selecting a branch?

fixes: #1160

[quintesse]

1. has been fixed
2. the user still has to click a button (or hit ENTER) to start the analysis, right?

I think the ideal solution would be to have the branch prefilled with "master" and only get the actual list of branch names the moment the user clicks the dropdown to see the list. In 99% of the cases people will just use master and don't need anything else, so perhaps we shouldn't waste any time waiting for a result that will be used only in 1% of the cases.

Something like this perhaps?:

[ia3andy]

@edewit I am closing the PR until you start working on this feature again ;-)