We previously used the window load event as a target to remove the splash. The issue with that is that it means we wait for images to download before we remove the splash.
Ember has a better method that we can use ready(). This PR triggers a custom discourse-ready when that happens and uses that as the baseline for removing the splash.
This PR also adds three new performance marks. discourse-ready, discourse-splash-visible, and discourse-splash-removed
These will help us keep track of performance.
Internal topic /t/65378/81