We use Backbone.Router and URLs to set the current view of the diff

viewer, controlling which revisions and pages are shown. Currently, this

is pretty basic -- we only need to worry about revisions in the path,

and a single query string option for the page. This means that any bits

of code updating this view state just needed to have the router navigate

to a new URL. Soon, though, we'll need more query string options, and we

weren't set up for this at all. This change aims to fix that.
There's now a central _navigate() method, which takes care of setting

the URL state based on the options provided and the existing page state.

It's the source of truth on how page URLs set within the view should

look. All call sites now call into this, instead of working with URLs

themselves.
The router itself now supports arbitrary query strings. This means

switching to our own regex, instead of using their capture groups (which

don't actually work well for more general usage -- there's lots of

workarounds online, but this seems to be the best option). While the

regex is a bit more complicated, the body of the handler is now a bit

simpler, and will be easier to expand with more diff loading options

going forward.
DiffViewerPageModel was updated to use the new Djblets.buildURL()

function for constructing the URL used to load the diff context, which

will also help us with expanding usage soon.
Unit tests were added that test the various navigation combinations,

both in a generic way (testing _navigate()) and more specific ways

(testing the handlers for revision/page selection, anchor

navigation, and initial page states).

All unit tests pass.
Manually tested changing revisions (testing all combinations of single

and interdiff revisions), changing pages, navigating anchors, and setting

initial pages. The page state and the current URL always reflected what I

expected to see.