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Fix spurious KaTeX errors in the exercise editor (#592)

## Summary:
Previously, any use of `\ce` in a TeX expression resulted in the
exercise editor displaying a KaTeX error, even though the expression
preview rendered fine (and was in fact being rendered by KaTeX). This
was because the preview and the error pane were using different KaTeX
configurations. The preview had the mhchem extension installed, and the
error pane didn't.

This commit fixes the issue by adding mhchem to the error checker's
KaTeX.

Issue: https://khanacademy.atlassian.net/browse/LC-997

Test plan:

Install the dev version of Perseus in webapp.

Edit an exercise. Add the following TeX somewhere:

`$\ce{Ba}$`

You should see "Ba" rendered in an upright font in the preview. The
"KaTeX Errors" pane should not display.

Change the TeX to `$\mbox1$`. The "KaTeX Errors" pane *should* display
(KaTeX doesn't have an `\mbox` command). A "1" will be rendered in
the preview by the MathJax 2 fallback renderer.

Author: benchristel

Reviewers: nixterrimus

Required Reviewers:

Approved By: nixterrimus

Checks: ✅ finish_coverage, ✅ Publish npm snapshot (ubuntu-latest, 16.x), ✅ Extract i18n strings (ubuntu-latest, 16.x), ✅ Check builds for changes in size (ubuntu-latest, 16.x), ✅ Jest Coverage (ubuntu-latest, 16.x), ✅ Lint, Typecheck, and Test (ubuntu-latest, 16.x), ✅ Cypress Coverage (ubuntu-latest, 16.x), ✅ Check for .changeset file (ubuntu-latest, 16.x), ✅ gerald

Pull Request URL: https://github.com/Khan/perseus/pull/592

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