Feature/activity refactor

Various fixes, improvements, and refactoring.  Refactoring is to move things out of main activity, which has gotten rather large, and is difficult to test directly.

-  Fix missing `FragmentManager` initialization for backstack clearing in tabs adapter.
-  Factor out `TabsAdapter` to its own class.
-  Factor out location management methods from main activity to `VoterInfo` model.
-  Alert user if address not geocoded, instead of showing map with marker at (0, 0).