Display debug mode indicator in menu drawer

Without this there isn't an easy way to determine if the app is in debug
mode or not. This change was prompted by realizing halfway through
testing the app that it was in release mode rather than debug mode.
Fortunately I didn't cause any damage in the meantime.
Merge branch 'display-debug-mode' into offline-guides

Conflicts:
	App/res/values/strings.xml
Increase guide progress update interval

Because the progress is updated via BroadcastReceivers rather than
sending out an update, it doesn't matter what the value in the DB is
unless the sync is cancelled halfway through. Even 10 seconds may be too
short...