Revert membership of smart group

In a contact's "Groups" tab you can see when somebody has been Added, Pending or Removed from a smart group. However there is no means to make their membership revert back to the smart group criteria. This may mean that somebody is permanently added or removed, when this may not be appropriate.

This patch adds a [ Delete ] option to the right of Current [ Remove ], Pending [ Remove ] and [ Rejoin group ]. It removes the appropriate entry from civicrm_group_contact, thereby reverting to the smart group criteria. For non-smart groups, the contact will be not present in the group.

The documentation should be updated to describe precisely what the [ Delete ] option does.

A new entry is also appended to civicrm_subscription_history. This table needs altering to add the enum option 'Deleted' as per the following. This code is not included in the patch.

ALTER TABLE `civicrm_subscription_history` CHANGE `status` `status` ENUM( 'Added', 'Removed', 'Pending', 'Deleted' ) CHARACTER SET utf8 COLLATE utf8_unicode_ci NULL DEFAULT NULL COMMENT 'The state of the contact within the group'