Clean up reviews/urls.py.
This cleans up reviews/urls.py by separating out rules with common

prefixes into urlpatterns that can be included.
There are still some prefixes that remain across rules within a

urlpatterns block (the review e-mail previews), but those seemed to make

more sense kept together.

Tested all the review request and diff viewer pages, including diff fragments,

interdiffs (navigating to the page, not just dragging the slider), and e-mails.
Unit tests pass.