Quick configure for external services
Created by: beyang
Addresses https://github.com/sourcegraph/sourcegraph/issues/2057 by adding "Quick configure" buttons on the "add external service" page and introducing an intermediary page where the user selects what type of external service to add (this replaces the external service kind dropdown).
TODO
-
Fix header margin on "Add/Update External Service(s)" pages. (Should be flush with top of left menu.) -
Add snapshot unit tests.