The e-mail preview views were shadowing the format builtin via a

argument which indicated the format to render the e-mail as: plain text

or HTML. In order to not shadow format, it has been renamed

message_format and all URLs and tests have been updated appropriately.
In addition, the preview password change e-mail view has been updated to

use message_format from the URL instead of extracting it from

request.GET.

Ran unit tests.