CRM-13422 - ActionMappings - getRecipientTypes(), getRecipientListing()

CRM-13422 - ActionMappings - getRecipientTypes(), getRecipientListing()

Changes:
 * Declare getRecipientTypes() before getRecipientListing() because it's easier to read in that order.
 * Tweak docblocks
 * Split combined/legacy functions (Civi\ActionSchedule\Mapping) into simpler per-entity functions (CRM_Activity_ActionMapping, CRM_Event_ActionMapping).
 * For "Contribution Type" reminders, allow targetting to soft creditors.
 * For "Contribution Page" reminders, don't bother with soft-creditors. If someone paid through a self-service form, then I'm not sure we have soft-creditors.