The GitLab API v4 support wasn't factoring in pagination for the

branches API (or the groups API, but that's currently only used on v3).

This means that if more than 100 branches are defined, only those first

100 will be shown in the New Review Request page.
This switches our implementation to use the standard pagination support.
It also cleans up the cod around checking the Link header to meet our

coding standards. Note that capitalization has changed for the Link

key in this code, but in practice the underlying code normalizes key

lookups, so this doesn't change any logic. It just makes it easier to

test against.
Unit tests have been added to check that paginated branches work

correctly.

Unit tests pass.