Passage: [[passage-ref]]s for automatic line numbers

Summary:
Adds a [[passage-ref]] widget that allows content creators to reference
specific sections in a passage (denoted by {{double curly-braces}}).

This breaks formatting within referenced sections--we'll need to make
an inline renderer (that doesn't output a paragraph) to get that back.

Test Plan:
Played around with
[this item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20passage%201%5D%5D%5Cn%5CnOn%20%5B%5B%E2%98%83%20passage-ref%201%5D%5D%2C%20the%20author%20posits%20that%3A%5Cn%5Cn%5B%5B%E2%98%83%20radio%201%5D%5D%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fda8df81c78b22f5c69d477d8eabfb583968eaf84.png%22%3A%7B%22width%22%3A400%2C%22height%22%3A70%7D%2C%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fb59fc02ca1aae800977b8793ed22f647a1aa75ee.png%22%3A%7B%22width%22%3A425%2C%22height%22%3A150%7D%7D%2C%22widgets%22%3A%7B%22passage%201%22%3A%7B%22type%22%3A%22passage%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22passageText%22%3A%22The%20pioneers%20of%20the%20teaching%20of%20science%20imagined%20that%20its%5Cnintroduction%20into%20education%20would%20remove%20the%20conventionality%2C%20artificiality%2C%20and%20%7B%7Bbackward%20lookingness%7D%7D%20which%20were%20characteristic%3B%20of%20classical%20studies%2C%20but%20they%20were%20gravely%20disappointed.%20So%2C%20too%2C%20in%20their%20time%20had%20the%20humanists%20thought%20that%20the%20study%20of%20the%20classical%20authors%20in%20the%20original%20would%20banish%20at%20once%20the%20dull%20pedantry%20and%20superstition%20of%20mediaeval%20scholasticism.%20The%20professional%5Cnschoolmaster%20was%20a%20match%20for%20both%20of%20them%2C%20and%20has%20almost%5Cnmanaged%20to%20make%20the%20understanding%20of%20chemical%20reactions%20as%20dull%20and%20as%20dogmatic%20an%20affair%20as%20the%20reading%20of%20Virgil%27s%20Aeneid.%22%2C%22showLineNumbers%22%3Atrue%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%2C%22passage-ref%201%22%3A%7B%22type%22%3A%22passage-ref%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22passageNumber%22%3A1%2C%22referenceNumber%22%3A1%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%2C%22radio%201%22%3A%7B%22type%22%3A%22radio%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22choices%22%3A%5B%7B%22content%22%3A%22Line%20breaks%20are%20evil%22%2C%22correct%22%3Afalse%7D%2C%7B%22content%22%3A%22Underlines%20are%20evil%22%2C%22correct%22%3Afalse%7D%2C%7B%22content%22%3A%22Widgets%20are%20evil%22%2C%22correct%22%3Atrue%7D%5D%2C%22randomize%22%3Afalse%2C%22multipleSelect%22%3Afalse%2C%22displayCount%22%3Anull%2C%22noneOfTheAbove%22%3Afalse%2C%22onePerLine%22%3Atrue%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D)
It seemed to be working.
I haven't tested grading.

Reviewers: charlie, michelle

Reviewed By: charlie, michelle

Subscribers: elizabeth, marcos, eater

Differential Revision: http://phabricator.khanacademy.org/D11888