Bump ezyang/htmlpurifier from 4.17.0 to 4.18.0 (#91)

Bumps [ezyang/htmlpurifier](https://github.com/ezyang/htmlpurifier) from 4.17.0 to 4.18.0.
- [Release notes](https://github.com/ezyang/htmlpurifier/releases)
- [Changelog](https://github.com/ezyang/htmlpurifier/blob/master/NEWS)
- [Commits](ezyang/htmlpurifier@v4.17.0...v4.18.0)

---
updated-dependencies:
- dependency-name: ezyang/htmlpurifier
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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