Change rules for role dates

Dates can now be changed after they have started and ended. This means
that there is no situation which needs disabling the input fields, and
disabling has been removed.

Start and end date can now be in the past. The following rules apply:
 - Start dates can be any date in the past, and no more into the future
   than the max days property of the role type.
 - End dates follow the same rules
 - End dates must be equal to or later than start date.

Notification publishing has been reviewed to ensure duplicate
notifications are not created when start or end date is today.

Resolve: GREG-148