loading screen uses low resolution favicon

The loading screen is shown when a user is logged in, but before the main web app is shown. It can also be seen when the page is reloaded. Currently, the loading screen uses favicon.ico. This file is usually only small (recommended 32px or 64px square image). Instead, one of the more high resolution pictures could be used for the loading screen, which are already available in the assets directory. These files are larger and should look better at the size they are shown in the loading screen.