Show synced repositories in add/edit external services UI

Created by: beyang

Show synced repositories in when adding/editing an external services in this area:

Feature notes:

• Live-update list of repos ideal when user changes config (but need to check whether this is possible). Otherwise, list should update after user clicks "Update external service" and "Add external service" UI should perhaps include another button that triggers an update.

Related / blocked on:

• https://github.com/sourcegraph/sourcegraph/issues/2574
• https://github.com/sourcegraph/sourcegraph/issues/2025

TODO:

• Chat with @keegancsmith and @tsenart about how to query server for repositories associated with the given external service.
• Chat with @francisschmaltz about design

cc @sqs