Why you should forget everything you learned about footnotes

Summary:
Change the footnote numbering syntax in the footnotes field
from `$^1$` to `^`, to match the syntax within passages.

Test Plan:
Load [this item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20passage%202%5D%5D%5Cn%5Cn%5B%5B%E2%98%83%20passage%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%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%22It%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%5E%20is%20that%5E%20it%20has%20a%20more-or-less%20normal%20distribution%20of%20letters%2C%20as%20opposed%20to%20using%20%27Content%20here%2C%20content%20here%27%2C%20%7B%7B__making%20it__%7D%7D%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%5E%20hi%20afkjdaj%20afjaf%20klfjfas%20lkfadsjaf%20ljafdkl%20afdj%20falk%20afjklafd%20lafdskljfads%20lkafdsjfa%20dslkafdsj%20lfadskj%20fadslkfajsd%20aflsdkj%20jfasdljafsdlak%20fsdj%20afsdlkjaf%20djklafsd%20jafdsjklafsd%20lkjfadsjklafsd%20lkfasdj%20klafds%20kjlafds%20jkafldsjk%20l%5Cn%5Cn%5E%20bye%22%2C%22showLineNumbers%22%3Atrue%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%2C%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%22%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)

See footnotes with numbers 1 and 2, without using katex syntax.

Auditors: michelle