Continue line numbers between passages

Summary:
We made it so that when you have multiple passages in a question the
line numbers in the second passage will start where the line numbers
in the first passage left off.

Test Plan:
Open 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%5CnPassage%21%21%21%5Cn%5Cn%5B%5B%E2%98%83%20number-line%201%5D%5D%5Cn%5Cn%5Cn%5B%5B%E2%98%83%20passage%202%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%22passageTitle%22%3A%22%22%2C%22passageText%22%3A%22stry.%20Lorem%20Ipsum%20has%20been%20the%20industry%27s%20standard%20dummy%20text%20ever%20since%20the%201500s%2C%20when%20an%20unknown%20printer%20took%20a%20galley%20of%20type%20and%20scrambled%20it%20to%20make%20a%20type%20specimen%20book.%20It%20has%20survived%20not%20only%20five%20centuries%2C%20but%20also%20the%20leap%20into%20electronic%20typesetting%2C%20remaining%20essentially%20unchanged.%20It%20was%20popularised%20in%20the%201960s%20with%20the%20release%20of%20Letraset%20sheets%20containing%20Lorem%20Ipsum%20passages%2C%20and%20more%20recently%20with%20desktop%20publishing%20software%20like%20Aldus%20PageMaker%20including%20versions%20of%20Lorem%20Ipsum.%20asdmnad%2Casas%20a%20kjdsafdskjf%20kshfd%20askjdahfs%20ds%20sdkhfksj%20hs%20dfj%20hsfs%20dfkshdf%20s%5Cn%5CnIt%20is%20a%20long%20established%20fact%20that%20a%20reader%20will%20be%20distracted%20by%20the%20readable%20content%20of%20a%20page%20when%20looking%20at%20its%20layout.%20The%20point%20of%20using%20Lorem%20Ipsum%20is%20that%20it%20has%20a%20more-or-less%20normal%20distribution%20of%20letters%2C%20as%20opposed%20to%20using%20%27Content%20here%2C%20content%20here%27%2C%20making%20it%20look%20like%20readable%20English.%20Many%20desktop%20publishing%20packages%20and%20web%20page%20editors%20now%20use%20Lorem%20Ipsum%20as%20their%20default%20model%20text%2C%20and%20a%20search%20for%20%27lorem%20ipsum%27%20will%20uncover%20many%20web%20sites%20still%20in%20their%20infancy.%20Various%20versions%20have%20evolved%20over%20the%20years%2C%20sometimes%20by%20accident%2C%20sometimes%20on%20purpose%20%28injected%20humour%20and%20the%20like%29.%5Cn%22%2C%22footnotes%22%3A%22%22%2C%22showLineNumbers%22%3Atrue%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%2C%22number-line%201%22%3A%7B%22type%22%3A%22number-line%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22range%22%3A%5B0%2C10%5D%2C%22labelRange%22%3A%5Bnull%2Cnull%5D%2C%22divisionRange%22%3A%5B1%2C12%5D%2C%22labelStyle%22%3A%22decimal%22%2C%22labelTicks%22%3Atrue%2C%22numDivisions%22%3A5%2C%22tickStep%22%3Anull%2C%22snapDivisions%22%3A2%2C%22correctRel%22%3A%22eq%22%2C%22correctX%22%3Anull%2C%22initialX%22%3Anull%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%2C%22passage%202%22%3A%7B%22type%22%3A%22passage%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22passageTitle%22%3A%22%22%2C%22passageText%22%3A%22%5Cn%20%5CnContrary%20to%20popular%20belief%2C%20Lorem%20Ipsum%20is%20not%20simply%20random%20text.%20It%20has%20roots%20in%20a%20piece%20of%20classical%20Latin%20literature%20from%2045%20BC%2C%20making%20it%20over%202000%20years%20old.%20Richard%20McClintock%2C%20a%20Latin%20professor%20at%20Hampden-Sydney%20College%20in%20Virginia%2C%20looked%20up%20one%20of%20the%20more%20obscure%20Latin%20words%2C%20consectetur%2C%20from%20a%20Lorem%20Ipsum%20passage%2C%20and%20going%20through%20the%20cites%20of%20the%20word%20in%20classical%20literature%2C%20discovered%20the%20undoubtable%20source.%20Lorem%20Ipsum%20comes%20from%20sections%201.10.32%20and%201.10.33%20of%20%5C%22de%20Finibus%20Bonorum%20et%20Malorum%5C%22%20%28The%20Extremes%20of%20Good%20and%20Evil%29%20by%20Cicero%2C%20written%20in%2045%20BC.%20This%20book%20is%20a%20treatise%20on%20the%20theory%20of%20ethics%2C%20very%20popular%20during%22%2C%22footnotes%22%3A%22%22%2C%22showLineNumbers%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)
Note that the line numbers update when you update the passages, and
that you are able to add widgets and other content in between passages.

Reviewers: aria

Reviewed By: aria

Differential Revision: http://phabricator.khanacademy.org/D13918