Spike: Investigate branch based permissions

Created by: ryanslade

Some code hosts, for example Gerrit, use branch based permissions.

We should investigate the scope of work required to extend sub-repo permissions to support this.

We'll need to audit all the places where we currently check sub-repo permissions and confirm the work required to add branch information there if we don't already have it.

Looking at all the places where we use RepoContent is a good start.

We also need to spec out the changes required to the sub_repo_permissions table.