There are two new mixins added for generic views: One for working with

ETags, and one for more complex views that need to alter dispatch logic.
ETagViewMixin is a mixin that allows for computing an ETag for a generic

view, checking if the client already has a copy of the content based on

that ETag, and setting appropriate headers in the response. This supports

HTTP GET and HTTP HEAD (but currently no other methods).
PrePostDispatchViewMixin is useful for views that want to run logic

in the dispatch phase but also make use of mixin that add decorators to

dispatch(). Since the view's dispatch() would run before the ones

decoratored in the mixins, there wasn't a good way to override dispatch

behavior (calculating state, checking access permissions, or augmenting

responses).
This mixin solves this by splitting the dispatch() method into a

pre_dispatch() and post_dispatch(). When placed after mixins adding

decorators and before mixins altering dispatch() logic, views will be

able to get custom logic into the dispatch phase.
pre_dispatch() can calculate state and optionally return a response to

send directly to the client (useful for access checks).
post_dispatch() can take a response from the HTTP handler and alter

it. It can even return a custom response, if needed.

Unit tests pass.
Tested manually with some new views I'm writing.