team shifts: implement marking shifts for sale

[reynir]

A common occurence is that not enough people volunteer for shifts at
first, and then the few volunteers take all the shifts (too many). Then
late comers aren't able to take any shifts.

This commit implements a third state for shift assignment: for sale. A
shift marked for sale means the team member is able to take the shift if
need be, but would rather someone else takes the shift. Another team
member will be able to take shifts at the same time even if the required
number of people are met.

The current semantics is for-sale shifts are replaced eagerly. Another
possible implementation would be to only replace for-sale shifts if the
number of required people is met.

I have not tested this #yolo

[tykling]

This would be nice to get in, but it needs a rebase and needs to be properly tested. Are you still interested in working on this @reynir or should we try to find someone else to get it merged?

[reynir]

Thanks. At this time I'm not motivated enough to work on it, but it would be great if someone can bring it forward!

[tykling]

closing for now, feel free to reopen, or open an issue so the idea doesn't get lost 👍