Refactor form create to return proper status codes

If a form is created, it returns a 201.
If a form isn't created, because it already exists, return a 202.
Refactor FulcrumTable to follow the convention that a success returns a
table while a failure returns nil. Makes it easy to return the proper
status code.
Refactor FulcrumTable#existing_table
Make FulcrumTable#drop_table robust/follow convention
Make form update return proper status code
Make form delete return proper status codes