Allow API resource extra_data handling to be disabled per-field.
Review Request #6539 — Created Oct. 30, 2014 and submitted
WebAPIResource subclasses can now determine on a case-by-case basis
whether a user-provided extra_data.* field can be set in a request.This will allow for special handling of certain bits of extra_data by
Markdown-capable custom text fields.
Unit tests pass.
Tested this manually along with some upcoming changes and a custom field.