Lots of updates combined

Closed #111 (closed): Clear notifications functionality

Closed #110 (closed): Instead of showing time since notification, show text "time.since.notification"

Closed #108 (closed): Prevent sending shift (NWH) to shift market

Closed #107 (closed): Functionality to change shift date and slot from editor

Line manager receives email after somebody takes shift from market

Fixed time in notifications (model ShiftForMarket str)