Improve line number reference URLs for the text review UI.
For the text review UI, you can reference a line number by

appending #source/line123 to the URL. This improves it a bit by allowing

just #line123.
Since Backbone.Router isn't very smart when it comes to

pattern-matching, we have no way of specifying a mode just for line

numbers. The greediest entry seems to win, and that'd be the :viewMode

one. So, we need to transform in the router handler.

Tested referencing just the view mode, just the line number, and both.