Gitaly keyset pagination returns a limited number of
branches. We can use it instead of fetching all
branches and then paginating them manually.

Projects::BranchesByModeService is introduced.
It uses Gitaly page-token pagination in order to
optimally fetch branches.
The drawback of the page-token pagination is that
it doesn't provide an option of going to the previous
page of the collection.
That's why we need to fall back to offset pagination
when previous page is requested.