XWiki vulnerable to stored cross-site scripting via any wiki document and the displaycontent/rendercontent template

XWiki Platform is a generic wiki platform offering runtime services for applications built on top of it. Starting in version 2.2.1 until versions 14.4.8, 14.10.5, and 15.1RC1 of org.xwiki.platform:xwiki-platform-web and any version prior to 14.4.8, 14.10.5, and 15.1.RC1 of org.xwiki.platform:xwiki-platform-web-templates, any user who can edit a document in a wiki like the user profile can create a stored cross-site scripting attack. The attack occurs by putting plain HTML code into that document and then tricking another user to visit that document with the `displaycontent` or `rendercontent` template and plain output syntax. If a user with programming rights is tricked into visiting such a URL, arbitrary actions be performed with this user's rights, impacting the confidentiality, integrity, and availability of the whole XWiki installation. This has been patched in XWiki 14.4.8, 14.10.5 and 15.1RC1 by setting the content type of the response to plain text when the output syntax is not an HTML syntax.