graphqlbackend: add query `authorizedUserRepositories`
Created by: unknwon
Part of #7298, added GQL query authorizedUserRepositories
, which returns paginated list of repositories a user has access to.
Manually tested with some unit tests, regression tests will be added as the last PR of #7298.
It's safe to merge this PR (after review) alone because the site configuration option is not documented anywhere outside the source code.