Allow setting reminders to offline calendar events

We should decide if we want to allow setting reminders to offline events (right now you can't). This isn't trivial, it has impact in several places:

• Right now if you view an offline event, you're sent to the edit page directly (like we do in other places like forum). To allow adding reminders, we'd have to send the user to the view page instead.
• We'd have to set the reminders for offline events when they're created, edited or discarded.
• We'd have to remove reminders if an event is discarded when synchronizing.
• We'd have to move reminders from the temporary event to the final event when created.