Add user interface to allow activities to connect to multiple contacts

HRD requirement: Possibility of attaching case and activity to multiple 'Regarding' and 'To' fields

We need user interface to be able to pick multiple activity_targets and activity_assignments to single activity record. That would probably be done best using some Ajax widget.

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Original issue:

http://forum.civicrm.org/index.php/topic,2933.0.html

CiviCRM provides capability to to assign multiple target contacts to a single activity, but the UI doesn't provide the option.

Issue opened to track work commenced during CiviCRM Bootcamp Melbourne