Bug 837671 - [calendar] Implement new startup loading events

I don't think `moz-app-visually-complete` and `moz-content-interactive` have anything to do with the router. I think what we want to know for these is whether we've picked up the busytimes from the database and rendered them in the month view. Knowing when the _last_ route has been added to the pagejs routetable is (I think) necessary for `moz-app-loaded`, but we know that at the end of `App#init` and don't need the `_firstViewLoaded` variable I think. The month view html lives in `index.html`, so (since all of this code is running after `window.load`) once we add the `TimeHeader`, tab list, and load the busytimes from idb, the app should be visually loaded and (immediately thereafter) interactive. Does that make sense?