The e-mail preview views perform the same checks that the other review

requests perform, so it's a good opportunity to use the new

ReviewRequestViewMixin. This change converts the e-mail preview views to

use this. It also introduces a handy base class, BasePreviewEmailView,

that wraps the decorator and makes it easy to construct a class-based

view around it.
Potentially we may choose to convert the decorator entirely to the

class-based view, since it's now only being used for this purpose, but

this change keeps the decorator around.
Unit tests have been added for the new base view, and existing tests for

the e-mail previews continue to work.

Manually tested each view.
Unit tests pass.