Fix a regression in HTML escaping for text area fields.
Text area fields were no longer escaping the HTML-unsafe characters

contained in the source text, either when dealing with plain text or

Markdown escaping.
We now escape the text in the base TextAreaField after normalizing it.

Added some HTML-unsafe characters, reloaded the page, and checked the output.
Turned off the rich-text for the draft, and checked again. Also properly escaped.