Add more missions

[lfuelling]

Is your feature request related to a problem? Please describe.
There are currently only four missions (not counting the tutorial). There should be more.

Describe the solution you'd like
More missions.

Describe alternatives you've considered
I've considered just leaving it at this stage, but this is way too boring.
Another alternative I have considered is PvP hacking but that's a thing for post v1.0...

Additional context
Commit b3b5eae92 shows all the necessary stuff to add a mission.