[WIP] Add Ninja Forms support

[zackkatz]

There are items that aren't implemented yet.

Some background:

DataKit does filtering, searching, and sorting through our own single REST endpoint. We then use native plugin functions to generate the response using the get_data_ids() method.

I spent many hours trying to pass through the search and filter settings into the Ninja Forms REST API handler (using rest_do_request()), but I kept hitting walls. I then tried cloning over some methods from Ninja Forms into the class, but the amount of complexity that introduced was unacceptable.

I decided to fall back on a very basic CPT search that was inspired by the Ninja Forms code…and then ask for assistance.

Things that still need to be implemented

• Sorting by field (ASC/DESC)
• Searching all fields (as implemented in the NF Submissions screen)
• Replace get_data_ids() with a native function that allows passing search values
  • Update the count() method, when implemented
• Verify capability for deleting submissions (is delete_posts the correct capability to check?)