Provide useful lists of URL names for extensions.
This defines some lists of common URL names that extensions can use for

the various apply_to fields. These fall into three categories:
1. Diff viewer pages

2. Pages with reviewable content.

3. Pages showing the review request box with fields.
These are not yet documented, but will be in some of the new docs work

I'm putting together.

Used them in an extension for JS extension initialization. Only saw the

initialization on those pages.