The force-text-type= parameter in requests is useful for returning text

in a different format, but there are times when the client would need

the raw text along with this.
Now, the client can also send an include-raw-text-fields=1 parameter,

which will embed a raw_text_fields dictionary in the object's payload

containing an entry for each text field. Each entry provides the raw

version of the text. This is optional, in order to prevent the default

response from ballooning in size.
On an implementation level, this is done by removing all the custom

serializers, and instead adding some metadata to the resource's fields

list for each field that supports text types. MarkdownFieldsMixin's

serialize_object then handles embedding these raw fields in the payload

on request.

Tested manually wit and without the new parameter.
Unit tests pass.