List branches for a project, with filtering

A new activity tab for a project, shows branches in that project's repos, with filtering.
Yes. I think so too.

For this case, I think I'll retrieve from the repos endpoint, that contain nested branches.

But while trying to do this one and the previous story "quickly and naively" (trying to use existing API only if possible to focus solely on the UI bits) I found myself a bit uncomfortable with the current API design - I think it does not really reflect resource structure (repos into projects etc  e.g. as would be if we did `/project/123/repos` instead of `/repo?project=123` as we do now). Maybe something to chat about.