TemplateHook did some hard work in order to figure out the URL name to

match for applies_to. Turns out that Django provides the URL name for

us, through request.resolver_match.url_name. Since this is more

reliable, we can use this directly.
The applies_to support is pretty handy, and even though it's now

simpler, it's best not to have to re-implement it. This change pulls it

out into a mixin that can be easily used by other hooks.

Unit tests pass.