Attendee interface/connections/mutations added. Order interface added.

[kidunot89]

Summary

• Implements #36
• Adds Attendee Interface type along with the attendee connections from RootQuery and Event.
• Adds OrderInterface (Couldn't rename WooGraphQL's Order type) interface type.
• Adds Attendee data field for query attendee meta.
• Adds attendees field to Order type
• Adds registerAttendee mutation
• Adds updateAttendee mutation
• AttendeeQueriesTest and AttendeeMutationsTest implemented.