Do not display login form while loading #2802
When loading Graylog, we currently display the login form while waiting for some resources to load, namely:
Those resources sometimes take a bit to load, and seeing the login form is confusing, as seen in #2770. This PR introduces a loading page that will avoid the confusion of seeing the login form on the screen.
I decided to use the same design used in our login page, as this provides a good transition between the loading and log-in page, which may be show directly before and/or after each other.