Citizen allows stored XSS in user registration date message

Citizen is a MediaWiki skin that makes extensions part of the cohesive experience. Various date messages returned by `Language::userDate` are inserted into raw HTML, allowing anybody who can edit those messages to insert arbitrary HTML into the DOM. This impacts wikis where a group has the `editinterface` but not the `editsitejs` user right. This vulnerability is fixed in 3.3.1.