Bump showdown from 1.8.6 to 1.9.1 in /front-end (#551)

Bumps [showdown](https://github.com/showdownjs/showdown) from 1.8.6 to 1.9.1.
- [Release notes](https://github.com/showdownjs/showdown/releases)
- [Changelog](https://github.com/showdownjs/showdown/blob/master/CHANGELOG.md)
- [Commits](showdownjs/showdown@1.8.6...1.9.1)

---
updated-dependencies:
- dependency-name: showdown
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

front-end/package.json

@@ -49,7 +49,7 @@
     "mockjs": "1.0.1-beta3",
     "normalize.css": "7.0.0",
     "nprogress": "0.2.0",
-    "showdown": "1.8.6",
+    "showdown": "1.9.1",
     "sortablejs": "1.7.0",
     "vue": "2.6.0",
     "vue-i18n": "7.3.2",