Use popup balloons for parish details on Map

Overlays, when tapped, currently open up a dialog box with the options of 'Details' or 'Dismiss' — the iOS way of doing things is much simpler and more user-friendly, where a tap brings up a little bubble of information with a details disclosure icon.

Google's Places app also does it similarly.

We can use the https://github.com/jgilfelt/android-mapviewballoons library to open our text and snippets in popup balloons attached to the overlay's location, but it will take a little effort to make sure things don't get messed up elsewhere, or an accidental double-tap doesn't cause any problems.