add About screen

closes #144 (closed)

@donuts In this MR the last update section shows when the app was last updated on the device, not when the app was last released. The reason for it is that the former is prone to errors, since we may have tagged a release at a particular date, but it might take a while afterwards until the release was accepted by google. Also F-Droid builds are known to take several days. In both cases the app stores would show a different publishing date than the app itself.

The review of this MR has less priority than those for the bug fixes.