Option to pass an Event ID into the Register for Event form (standalone and contact/view modes)

Currently there is no way to 'preload' a specific event when navigating to the Event Registration form (CRM/Event/Form/Participant.php). If the event can be preloaded, then admins can create specific links in their menu or blocks to facilitate in-person/offline registrations.

1. Define / implement an addtional query string parameter for event id which can be added to either of these URLs
civicrm/participant/add?reset=1&action=add&context=standalone
civicrm/contact/view/participant?reset=1&action=add&context=participant&cid=102

2. Modify setDefaults() so that the following values are loaded when form is rendered:

• set Participant Status to 'Registered' by default (this always should be the default, regardless of whether an event is pre-loaded)
• set Participant Role to the value of civicrm_event.default_role_id if this is not NULL.
• load event fee block / priceset / custom data for this event

3. Add 'Register Participant' action to the Manage Events selector row actions ('more' pop-up menu) which links to participant form in standalone mode passing the event id. (actionLinks in Event/Page/ManageEvent.php).