Unknown form field will default to a TextField column

If a form create comes in with an unknown field type, we’ll create a
column for that field and treat it as a text field.
Move Extractors classes into a class namespace, not module

This is for a coming refactoring.
Pull list of Extractors out into own class

Matches the way schemas are handled.
Refactor default schema class

Makes it more readable.
Encode unknown record form values as JSON.

This makes it possible to accept unknown form fields and still put them
into the database as text.