fix: refresh list of extensions when viewing a repository
Created by: chrismwendt
This mitigates a bug where when you added a repository and a language server was automatically started then navigate to a file, the corresponding language extension was not active because the webapp's list of extensions was stale.
Fixes https://github.com/sourcegraph/enterprise/issues/13654
This PR does not need to update the CHANGELOG because this mitigates an unreleased bug.