Expression: Fix editor for old items (without tex)

Test Plan:
Load [this item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22The%20graphs%20of%20the%20functions%20%24f(x)%24%20and%20%24g(x)%24%20are%20given%20below%2C%20with%20their%20intersection%20points%20marked.%20One%20of%20the%20functions%20is%20linear%2C%20and%20the%20other%20is%20an%20exponential%20function%20of%20the%20form%3A%5Cn%5Cn%24%5C%5Cqquad%5C%5Clarge%7B%20a%20-%20r%5E%7B%5C%5Clarge%20x%7D%7D%5C%5Cquad%5C%5Cnormalsize%7B(r%3E0).%7D%24%5Cn%5Cn**Write%20the%20formulas%20of%20the%20functions.**%5Cn%5Cn%24f(x)%3D%5C%5Cmkern-18mu%24%20%7C%20%5B%5B%E2%98%83%20expression%201%5D%5D%20%7C%20%24g(x)%3D%5C%5Cmkern-18mu%24%20%7C%20%5B%5B%E2%98%83%20expression%202%5D%5D%5Cn-%3A%20%7C%20%3A-%20%7C%20-%3A%20%7C%20%3A-%5Cn%7C%7C%7C%5Cn%5Cn!%5B%5D(https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F14e840be109aeabcbc580e593cfef2541c78a2b3.png)%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F14e840be109aeabcbc580e593cfef2541c78a2b3.png%22%3A%7B%22width%22%3A425%2C%22height%22%3A421%7D%7D%2C%22widgets%22%3A%7B%22expression%201%22%3A%7B%22type%22%3A%22expression%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22value%22%3A%223-2x%22%2C%22form%22%3Afalse%2C%22simplify%22%3Afalse%2C%22times%22%3Afalse%2C%22functions%22%3A%5B%22f%22%2C%22g%22%2C%22h%22%5D%7D%7D%2C%22expression%202%22%3A%7B%22type%22%3A%22expression%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22value%22%3A%223-2%5Ex%22%2C%22form%22%3Afalse%2C%22simplify%22%3Afalse%2C%22times%22%3Afalse%2C%22functions%22%3A%5B%22f%22%2C%22g%22%2C%22h%22%5D%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22Fill%20in%20the%20blanks.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Atrue%7D%2C%22hints%22%3A%5B%7B%22content%22%3A%22We%20can%20determine%20which%20function%20is%20linear%20by%20looking%20for%20the%20linear%20graph.%20That%20would%20be%20the%20graph%20of%20%24f(x)%24%2C%20so%20this%20is%20the%20linear%20function.%20Therefore%2C%20%24g(x)%24%20is%20the%20exponential%20function.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22The%20linear%20function%20is%20%24f(x)%3Dmx%2Bb%24%2C%20where%20%24m%24%20is%20its%20slope%20and%20%24b%24%20is%20its%20%24y%24-intercept.%20%20Its%20graph%20passes%20through%20the%20points%20%24(%5C%5Cgreen%7B1%7D%2C%5C%5Cblue1)%24%20and%20%20%24(%5C%5Cgreen%7B2%7D%2C%5C%5Cblue%7B-1%7D)%24%2C%20so%20the%20slope%20is%3A%5Cn%5Cn%24%5C%5Cqquad%5C%5Cbegin%7Balign%7D%20m%26%3D%20%5C%5Cdfrac%7B%20%5C%5Cblue1-(%5C%5Cblue%7B-1%7D)%20%7D%7B%20%5C%5Cgreen%7B1%7D-%5C%5Cgreen2%7D%5C%5C%5C%5C%5C%5C%5C%5C%20%20%20%20%20%20%5Cn%20%26%3D-%5C%5Cdfrac%7B2%7D%7B1%7D%20%5C%5C%5C%5C%5C%5C%5C%5C%5Cn%26%3D-2.%20%5C%5Cend%7Balign%7D%24%5Cn%5CnThe%20slope%20is%20%24m%3D-2%24.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22The%20graph%20of%20%24f(x)%3Dmx%2Bb%3D-2x%2Bb%24%20%20passes%20through%20the%20point%20%24(2%2C-1)%24.%20%20This%20means%20if%20%24x%3D%5C%5Cred2%24%2C%20then%20%24y%3D%5C%5Cgreen%7B-1%7D%20%24%20and%20%5Cn%5Cn%24%5C%5Cqquad%5C%5Cbegin%7Balign%7D%20%5C%5Cunderbrace%7Bf(%5C%5Cred2)%7D%26%3D-2(%5C%5Cred2)%2Bb%5C%5C%5C%5C%5Cn%5C%5Cgreen%7B-1%7D~~%26%3D~-2(2)%2Bb%20%5C%5C%5C%5C%5C%5C%5C%5C%5Cn-1%20%26%3D-4%2Bb%20%5C%5C%5C%5C%5C%5C%5C%5C%20%20%20%20%5Cn%20b%26%3D3.%5C%5Cend%7Balign%7D%24%5Cn%5CnThe%20%24y%24-intercept%20is%20%24b%3D3%20%24.%5Cn%5CnThe%20linear%20equation%20is%20%20%24%20f(x)%3D-2x%2B3%20%24.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22The%20graph%20of%20%24g(x)%3Da%20-r%5E%7B%5C%5Clarge%20x%7D%24%20passes%20through%20the%20point%20%24(1%2C1)%24.%20%20This%20means%20that%20if%20%24x%3D%5C%5Cpink%7B1%7D%24%2C%20then%20%24y%3D%5C%5Cgreen1%24%20and%20%5Cn%5Cn%5Cn%24%5C%5Cqquad%5C%5Cbegin%7Balign%7D%20%5C%5Cunderbrace%7Bg(%5C%5Cpink1)%7D%26%3Da%20-%20r%5E%7B%5C%5Cpink%7B1%7D%7D%5C%5C%5C%5C%20%5Cn%5C%5Cgreen1~~%20%26%3Da-r.%5C%5Cend%7Balign%7D%24%5Cn%5Cn%5CnTherefore%2C%20%24~a%3Dr%2B1%24.%20%20%5Cn%5Cn%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22The%20graph%20of%20%24g(x)%3Da-%20r%5E%7B%5C%5Clarge%20x%7D%24%20also%20passes%20through%20the%20point%20%24(2%2C-1)%24.%20%20%20This%20means%20that%20if%20%24x%3D%5C%5Cpurple%7B2%7D%24%2C%20then%20%24y%3D%5C%5Cblue%7B-1%7D%24%20and%5Cn%5Cn%24%5C%5Cqquad%5C%5Cbegin%7Balign%7D%20%20%5C%5Cunderbrace%7Bg(%5C%5Cpurple%7B2%7D)%7D%26%3Da-r%5E%5C%5Cpurple%7B2%7D%5C%5C%5C%5C%5Cn%5C%5Cblue%7B-1%7D~~%26%3D~a-r%5E2.%5C%5Cend%7Balign%7D%24%5Cn%5CnTherefore%2C%20%24~a%3Dr%5E2-1%24.%5Cn%5CnLet's%20set%20the%20expressions%20of%20%24a%24%20equal%20to%20each%20other%20and%20solve%20for%20%24r%24%3A%20%20Then%5Cn%5Cn%24%5C%5Cqquad%5C%5Cbegin%7Balign%7D%20r%2B1%20%26%3Dr%5E2-1%20%5C%5C%5C%5C%5C%5C%5C%5C%5Cn%20%20%20r%5E2-r-2%26%3D0%20%5C%5C%5C%5C%5C%5C%5C%5C%5Cn%20%20%20%20(r-2)(r%2B1)%26%3D0.%20%5C%5Cend%7Balign%7D%24%5Cn%5CnIt%20follows%20that%20%24r%3D2%24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locally

Auditors: joel