Allow force-text-type= to apply to custom fields in extra_data.

Review request and draft API resources now show custom review request
text field values in the requested text type, matching the behavior of
the description, testing_done, etc. fields.

This is done by having MarkdownFieldsMixin own the
serialize_extra_data_field function. It then asks the specific API
resource class to determine if a given field in extra_data supports
Markdown. If so, that field's text will be normalized.

ReviewRequestResource and ReviewRequestDraftResource both provide this
function and check the given field to see if it's Markdown-capable.

Reviewed at https://reviews.reviewboard.org/r/6420/