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.
Make all URL names consistent.

This changes all the URL names to use '-' instead of '_' as a separator.
The '-' is more common in Review Board, and is used exclusively in
Djblets.
Rename 'show_submitted' to 'show_closed' everywhere.

Since we're now providing 'Show/hide closed' instead of
'Show/hide submitted', it makes sense to keep the naming the same
everywhere. This change updates the field in Profile, the query
parameter, and the code in the datagrid to use the new naming.