Move towards being a single activity app

[veyndan]

Simplifying the lifecycle of the app to just be a single activity should help to significantly mitigate the prevalence of random crashes due to the entanglement of the fragment and activity lifecycles. The use of a single activity is recommended by Google, and the code should be migrated to use a single activity and multiple fragments.

Obviously, migrating the whole app in one fell swoop to a single activity would be insanely difficult. I'm envisioning that we slowly migrate over until we eventually have migrated to a single activity.