Remove app.router and rework signin and signout app notifications

app.router is no longer necessary since we moved to the sidebar, and hence it should be removed.

Additionally, the global signin and signout triggers can now be replaced by explicitly listening to the app.data.user model for changes.

More specifically, these changes:

- Removes app.router.
- Replaces the main view creation with SidebarApp (this was previously done in the router).
- Removes the global signin and signout triggers, replacing the listeners with new events on the app.data.user model.
- Removes the app.resetApp() function, replacing it by listening to the signout option on the app.data.user model.
- Adds tests for the code that has been changed

I suggest @n1k0 reviews this, especially the changes to the model, and the removal of the app.router.