mediawiki.skinning.content.externallinks removed in 1.39

[jdlrobson]

Impact: All custom external link icons (for PDFs) will be removed.

To fix this:

Remove the module mediawiki.skinning.content.externallinks

Optional:

Update skin.json to include "content-links-external": true, in the features array of your ResourceLoaderSkinModule (see https://github.com/wikimedia/mediawiki-skins-Wisky/blob/master/skin.json#L40 for an example)

Add the styles locally to the extension (to avoid breaking changes in future)