Support setting branch in settings file

[jaknor]

Is your feature request related to a problem? Please describe.
Everyone doesn't name their branch main.

Describe the solution you'd like
When adding a new source also prompt for the repo branch (can default to main). It appears share.js already expects there to be a branch, it is just never prompted when adding a new source. Potentially we should also import from the branch specified in the config?

Describe alternatives you've considered

Additional context