This adds a new query argument for the diff viewer and the diff context

API for filtering the list of files based on a provided list of

filenames or Unix-style file patterns. This allows users (or extensions

or other clients) to craft diff viewer URLs that show only a subset of

the files a person needs to review.
This is controlled through the ?filenames= query argument, which takes

the comma-separated list of filenames or patterns. This is parsed by

DiffViewerView into a list and then passed to get_diff_files(),

which will use it to filter each item. The JavaScript side of the diff

viewer then takes this in and uses it when constructing URLs and

navigating pages/revisions, ensuring the filtering is not lost during

navigation.

All Python and JavaScript unit tests pass.
Manually tested with various filters, checking to make sure the filters

were not dropped when changing revisions or pages.
Built the docs and checked for errors.