Preferences: Fix issues from 6d3c65b5b0

Preferences: Fix issues from 6d3c65b5b0

* Use .mw-preferences-messagebox instead of .successbox to
  avoid conflicts with other Special pages using '.successbox'
  as a class and so that the new class can be used to check
  for messages to replace with notifications
* Add logic to check for messages to replace using the new
  aforementioned class
* Use Html::rawElement and Html::element instead of Xml::tags
  to add the non-JS successbox
* Re-added <p> tags around non-JS successbox message that were
  accidentally ommitted in the original patch

Bug: T19496
Change-Id: I990667aa114d8201516bee6cb2ad22994de53c6c