Drop OSCAR_OFFER_BLACKLIST_PRODUCT functionality

It only prevents adding a product to a range; offers can be applied
nonetheless (despite the docs saying otherwise). The same functionality
can be achieved by overriding get_is_discountable, so it can be safely
removed.
Use plain textarea widget for email template editing

The dashboard communications app allows editing database-backed email
templates. The HTML body part had TinyMCE enabled, but I could not find
evidence that TinyMCE supports Django's template syntax. It definitely
caused an issue as reported in #1356.

Also use "fields" instead of "exclude" because that's good practice.

Closes #1356.