[MERGE] [FIX] mail: mail composer: Use a text field instead of a html field

html fields are sanitized just before saving data into the database.
In the composer, this cause the sanitization of the templated
message (for mass mailing messages) before the rendering, forbidding
the use of templated html links.
The html of the message is sanitized by message_post()

bzr revid: chs@openerp.com-20130416083812-jvqeivfxhdr32djh