Improve location settings prompts

[flibbertigibbet]

For geofencing to work, the device must have the "high accuracy" location setting enabled, which is something we can check for, but not a system permission that we can prompt for. So instead, if location settings do not allow geofencing, the app on first open will launch location settings and show a Toast message explaining what's needed.

A better interaction would be to use dialogs or some other message easier to read than the disappearing Toast message, and also to not prompt on every app open, as geofencing is not required for the app to work otherwise.

[jcahail]

Maybe gather feedback from beta.