Wrap parser output in <div class="mw-parser-output">

Wrap parser output in <div class="mw-parser-output">

This will allow CSS to target just the parser output, without also
accidentally targeting the edit form, diff tables, and so on.

Bug: T37247
Change-Id: If4eb5bf71f94fa366ec4eddb6964e8f4df6b824a
Depends-On: I330c6aa4aaee045614b1801ed34bc9e03be69650
Depends-On: I52a518fa44e017841fe78474012cd69823e0a41d