Add ability to send activity email for Add/Edit Activity Actions

Piggy backing on !82 (merged). This merge request add the ability to send an email to assigned contacts on a new or edited activity.

When Send Email is selected, all assigned contacts on a new activity will receive a notification email. For editing the triggering Activity, only newly assigned contacts will be send an email; this defers existing assigned contacts to what intended from the UI.

This is a similar feature to the AssignNEmail action, however gives the ability to send emails to specified contacts and not just the contact from the Trigger.

This change should be backwards compatible with actions with existing action, defaulting the send email flag to false.