Git service: add method to sync user access to remote repositories (#12016)
Extracted from
https://github.com/readthedocs/readthedocs.org/pull/11942.
Instead of getting a service for the user and manually syncing the
repositories, this abstracts syncing the access to the repositories in
another method. This is since with GH apps, syncing the access to the
repositories is done in a different way.