Update to wysiwyg for sending html

[JWardee]

Is your feature request related to a problem? Please describe.
Currently if a user sends an email using the mail catcher modal and ticks the "is html" box then pastes in their html in the wysiwyg editor (visual mode) then html is interpreted literally, this is correct but users may be wondering why their html isn't rendering correctly.

Describe the solution you'd like
When the "is html" checkbox is ticked, switch the wysiwyg editor out with a generic text box. Or force the wysiwyg editor to only allow the code tab.