Add loading animation to OAuth login page

Created by: temparus

When a tool takes long to show the page, redirected by the API from the OAuth login, the login page just stays there and the user does not have a visual feedback that the login process has started (it is only possible on the reload button of the browser to see that the page is loading).

We could add a little piece of JavaScript to replace the login button with a loading animation when the form is submitted.