team shifts: be able to mark shifts "for sale"

[reynir]

Is your feature request related to a problem? Please describe.
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.

Describe the solution you'd like
When a volunteer has a shift they should be able to mark it as up 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 slots that are otherwise full if there are any shifts for sale.

Additional context
About three years ago I made an implementation that I didn't test as setting up a dev environment was not straight forward. It may be useful to look at it if someone is to pick this task up. #882