Reschedule: allow to specify a start date

[jbenito]

Rescheduling is lacking an option to reschedule based on the first entry (session or contribution depending on the context) rather that the start time of parent (session or session block). For example we were supposed to start at 9:00 but decided to start at 14:00. After we made some mods to the session block duration and asked for rescheduling everything with a zero interval. As a result everything restarted at 9:00. It'd be great to be able to say reschedule with a zero interval without moving the start time of the first entry (session block).