CRM-15490 - Recurring Activities Using Core Recursion

The fixes allow activities to recur. Note that the recurring block only appears in update mode. This is inline with events, where repeat tab could only be configured in update mode. However for activities we might want it to make it available for create mode as well. 

Would be good to get some feedback / do some discussions with core team, as we don't have any clients / users using activity recursions. 

The fix also improves the test cases.
can't we just use empty() here?