Support sending force_text_type= in POST requests.

POST requests now support force_text_type=, in order to force the text
to be of a certain type. This mirrors the ?force-text-type= argument in
GET requests.

The resources that support this have been updated to take this argument
in their supported field lists.

The "html" text type is also no longer set as saveable.

Reviewed at https://reviews.reviewboard.org/r/6421/