Registrable Event CMS fields improvements


Squashed commits:
[3297e15] Moved cancelled registrations out of toggle field
Cancelled registrations weren’t showing while inside the toggle field,
plus being able to see both gridfields together makes management easier.
[561724c] Rearranged fields on Main tab
Put the title field back where it belongs.
Moved the Metadata fields below the event specific fields
[9b22d41] Added Send Invitation button to templates

Cherry picked some commits from @tardinha 's master branch

Included missing EventSendInvitationsButton, although it doesn't do anything.