Export/import full expeditions (including their state)

[on3iro]

as per this comment #328 (comment):

This allows users to transfer expeditions to other devices or even just make a backup "just in case".

To make this easily distinguishable from the config export, we probably should use a very obvious icon.
Also it should be very obvious, what the expedition import is and what the expedition config import is. This might confuse people.