Add external links controller
Add a controller that will be responsible to handle all external links in the code.
This will enable plugins to override the default behavior for all links that exist in our code:
Plugins will be able to change the url for all links, thus enabling things like internal documentation, or local caches.
This becomes more important once we start implementing links in both react and ruby code. Without this feature, we will need to duplicate link generation logic to both client and server.