Make the Project "Branches" page load faster

[jankeromnes]

Is your feature request related to a problem? Please describe

Currently, when you open a Project, you land on its "Branches" page.

This page synchronously fetches the GitHub/GitLab/Bitbucket API without any cache, and thus typically makes you wait 30-40s before showing anything interesting.

This is terrible, but thankfully it could be pretty easy to fix:

Describe the behaviour you'd like

• We could start by displaying the branches we already know about (e.g. we could fetch all N latest prebuilds, then derive the list of branches from them, thus making the "Branches" page as fast as the "Prebuilds" page -- and optionally enhance the list with unprebuilt branches later when the data arrives)

• We could also fetch the GitHub/GitLab/Bitbucket API ahead-of-time and cache the result

Describe alternatives you've considered

Additional context

This is also a customer-reported issue.