Make line selection in source code views less fragile and more consistent

Summary:
Depends on D19347. Ref T13105. See PHI565. The "highlight lines" behavior is interacting poorly with the new blame element in Diffusion.

Make the behavior a little simpler and hopefully more robust.

Test Plan:

• Clicked commit/revision links in Diffusion, saw the links get followed instead of the lines highlighted.
• Highlighted lines in Diffusion, saw just the line/code highlight instead of the whole thing.
• Highlighted lines in Paste and new-style Harbormaster build logs, saw consistent behavior.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19348