Improve usabilitty of MathQuill-extended graphs for people without the extension

[SinclaM]

If a user of Desmos Unlocked enables the extend MathQuill feature and uses symbols like nabla, partial, aleph, etc. to make a graph, and someone else opens that graph without the extension enabled, the graph will still run, but the symbols will be not be rendered.

It would be nice if people without Desmos Unlocked could still interact with the expressions of such graphs in a reasonable way, by seeing n_{abla} (n_abla) instead of nabla (∇).