[RFE] Ghost assignments cleanup

[grafuls]

Is your feature request related to a problem? Please describe.
With the new self scheduling capabilities, it might happen that new assignments are created but never pulled through into creating schedules for it. This would leave a lot of active assignments that would never be de-activated.

Describe the solution you'd like
Have a timeout for active assignments that do not get any schedules.

Describe alternatives you've considered
Change the default for the active field on assignments to False and have the assignment activated once the schedules are added.