Rework "standby blocks"

We used the "manually generated shifts" for more than standby:

• Helping out other teams with shifts not found in the Fahrplan
• Coordinator shifts
• Standby shifts

We should therefore consider the following changes:

• Rename the feature to "internal shifts"
• Allow the candidate/assignment flow for some or all internal shifts
• Add some sort of title which replaces the "Standby" branding for non-standby shifts
• Improve rendering for concurrent standby shifts

Additionally we could add a new "thing" called internal shifts and leave standby shifts alone.