[FIX] mass_mailing: avoid too long URIs by blacklisting fields in editor call

The call to the web_editor is made through an iframe with some get
parameters. While not really a problem in itself, this has the
disadvantage of generating long URIs. In some case, since all the
fields in the views are included as a GET parameter, URIs can be
generated that go up to several thousand characters. Most servers will
return a 414 for such URIs, preventing the edition of the mail
template in the view.

To avoid this problem, this commit extends the 'html_frame' widget and
blacklist 2 fields that can get too long once encoded.

Ideally, it would probably be better to stop sending all the fields and
simply add the *needed* fields one by one, but since it's difficult to
know exactly what fields might be needed and this is stable, I'm gonna
go with this 'workaround' but conservative fix.

opw-685646