Allow attaching multiple contacts to cases

Allow creating cases attached to multiple contacts. Functionality will be provided in following ways:
a/ Case adding screen will allow explicitly setting multiple contacts (in similar way as Activities allow it after recent changes)
b/ There will be a task on contact search results screen, which will allow registering a case for a group of selected contacts
c/ when new activity is being added from given case's context (using "Add activity" link on contact's case list), this new activitie's "Regarding" list will be populated with this case's contact list. So, if you have a case with contacts X, Y, Z attached, new activity screen will have the list "prefilled" with those X, Y, Z contacts, however, you will be able to change them any time, and no validation will be imposed on this list. In no other situation, the "Regarding" list will be modified or prefilled.

New joining table needs to be added (civicrm_case_contact), which will replace contact_id column from civicrm_case table.