Lost icons in debug window

[vaadin-bot]

Originally by @amahdy

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Add WSCDN and add Vaadin font icons in a maven project.

Open Debug window.

Result: debug window icons are lost.

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Imported from https://dev.vaadin.com/ issue #19417

[vaadin-bot]

Originally by @amahdy

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Error from Chrome console:

Font from origin 'http://wscdn.vaadin.com' has been blocked from loading by Cross-Origin Resource Sharing policy: The 'Access-Control-Allow-Origin' header contains the invalid value ''. Origin 'http://localhost:8080' is therefore not allowed access.