[Snyk] Security upgrade express from 4.13.4 to 4.21.0

[Randyblo7]

Snyk has created this PR to fix 3 vulnerabilities in the npm dependencies of this project.

Snyk changed the following file(s):

• node_modules/reveal.js/plugin/multiplex/package.json

Vulnerabilities that will be fixed with an upgrade:

	Issue	Score
	Cross-site Scripting SNYK-JS-EXPRESS-7926867	541
	Cross-site Scripting SNYK-JS-SEND-7926862	391
	Cross-site Scripting SNYK-JS-SERVESTATIC-7926865	391

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• Max score is 1000. Note that the real score may have changed since the PR was raised.
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