UI/accounts: adds a loading screen during account loading

- adds faded loading pages with a spinner animation
- starts and stops the load page during account creation and loading
- resizes the loading graphics when window is resized
- adds an event handler for DPI and scale factor changes
- removes stretching from welcome page image
- clears account creation alias box after account creation clicked

Change-Id: I5046e0bc820e91c8b2f91ca223534d93ddf916f1
Tuleap: #1010