graphqlbackend: add query `authorizedUserRepositories`

Created by: unknwon

Part of #7298 (closed), 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 (closed).

It's safe to merge this PR (after review) alone because the site configuration option is not documented anywhere outside the source code.