Convert "Help & Feedback" to "About Pilot Planner"

Change the content of the modal to explain the demo and signpost people
towards contacting us.
Don't get list of Locations from API, use static
Remove "this software is alpha" message
Add `pp-demo` as deployment target option

The reason I've added an option rather than changing the destination of
`pilotplanner` is that I can foresee cocking up a rebase in the future
and accidentally overwriting the real pilot planner with the demo one.
This way, we can make sure the demo Jenkins job specifies `pp-demo` so
it can't , hopefully, do that.