OutputPage: Add addParserOutputContent() for more fine-grained control

We previously had addParserOutput(), which added everything and did
some other magic, and addParserOutputNoText() which, as the same says,
added everything but the text.

I renamed addParserOutputNoText() to addParserOutputMetadata() and
created two more functions:

* addParserOutputText(): This is almost identical to adding the raw
  HTML, but calls the OutputPageBeforeHTML hook like other
  addParserOutput*() methods.

* addParserOutputContent(): Like addParserOutputText(), but also adds
  the ResourceLoader modules and variables associated with the parser
  output. This is important especially for some extensions like
  TemplateData or SyntaxHighlight which add styles to the page to
  enhance the display.

Change-Id: Iead541886fd1ccdbdf1cb06af71b34cd04644985