Turn off RichTextField in Settings

[domlysi]

Hey, is it possible to add a line in settings.py to disable the RichTextField in Admin for HTML Message in Email Templates?

It errors when I try to save the following a second time:
{{ object.dates|join:" / " }}

it parses it into the following after the save:

{{ object.dates|join:" / " }}

It's pretty annoying.

[selwin]

I think adding a feature to edit in plain text mode makes more sense. Would welcome a PR for this.