Passage: Measure lines automatically

Summary:
Counts the line numbers using jQuery instead of string logic.
This is useless in and of itself, but paves the way for getting
rid of whitespace: pre in the next diff.

Test Plan: See dynamic passage refs diff

Reviewers: charlie, michelle

Reviewed By: charlie, michelle

Subscribers: marcos, eater

Differential Revision: http://phabricator.khanacademy.org/D11887