web: add pages to CRUD user-added external services
Created by: unknwon
This PR contains the frontend changes for RFC 211:
- Refactor external services related pages to parameterized components.
- Allow users to manage external services in user settings.
- Respect a user-level feature flag from the backend indicating whether the user is allowed to add external services.
Huge thanks to @eseliger on (basically developed) frontend and corresponding tests!