Don't rely on ExternalServices -> config.repos

Created by: artemruts

Description

Fixes #20530 (closed) Fixes #20570 (closed) Fixes https://github.com/sourcegraph/sourcegraph/issues/20478

Changes the logic how we figure out what affiliated repos were already selected, before we used repos key inside the config JSON object but this proven not very safe, see https://github.com/sourcegraph/sourcegraph/issues/20478

Now will use additional userRepositories request to figure out which affiliated repos were already selected, this introduces yet another request but we have a more reliable way.

These changes should unblock the select all UI.