Home page and app contexts

[rashidkpc]

To support the addition of apps to Kibana we'd like to build a "app" screen that allows you to change the running context of Kibana.

The screen itself is a grid of icons. Clicking an icon brings you into the context of the app. Notice there are no tabs along the top. If we were to access the "Kibana" app from this screen we'd get the usual Discover, Visualize, Dashboard, Settings icons.

The app screen, and the button to access it should only be available if the user has other apps installed.

There does not need to be a way to add and remove apps right now, this is handled by #4204