Bring PageNotice extension into the modern era

* Splitting into three files, per current conventions
* Removed uses of deprecated wfMsgExt and wfEmptyMsg
* Switch to use ArticleView(Header|Footer) hooks, to ensure the notices
  only appear on articles (not on other parsed blocks of text)
* Other cleanup

Change-Id: I4b0bfbf02f997971927452114ec5d4ed5a629f90