Add styling for actionbar overflow menu, in case we ever need it.
First stab at a settings screen.

The code needs some cleanup and the checkboxes don't do anything. Also there's
no localization.
Settings localized and in sliding fragment.

The settings screen is now hosted inside a HorizontalSlidingFragment for
consistency with the rest of the app. All on-screen text has been localized. As
per the Android style guide, the settings action is always shown as an overflow
action.

The push notification subscription change is yet to be implemented.
Update push subscriptions when preferences change.