Support multiple cases when creating activity

In order to fully support HR-320 we need CRM_Case_Form_Activity to be able to create activities for multiple cases at once.
Currently the url argument "caseid" is a validated as single integer. Instead we need the form to support a comma separated list of case ids, e.g. caseid=3,7,19
During pre-processing it would need to validate the activity is appropriate to each case.
During post-processing it would then create 1 activity for each case.
Any special action such as changing the case status would also need to be done for every case.

To test this functionality, go to the case pipeline screen, select multiple contacts, and create an activity.