Make Paste source code line highlighting behavior more generic

Summary: Depends on D19163. Ref T13088. Increase the generality of this code so it can be shared with Harbormaster.

Test Plan: Clicked individual lines, clicked-and-dragged, etc., in Paste. Got sensible URI and highlight behaviors.

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19164