Convert from tag to includable special page, revamp tracking

As suggested on https://bugzilla.wikimedia.org/show_bug.cgi?id=65849,
converted the petition form from a tag to an includable special page.
This avoids misusing $wgRequest.

Petition can now be included in a page like this:
  {{Special:Petition/petitionname}}

Also removed the pagetitle and linksource tracking, as these needed
external context and weren't actually all that useful. Instead can now
include a tracking source parameter which will be recorded with signatures:
  {{Special:Petition/petitionname/source}}

Change-Id: If64de462aaca880df34d3eef81f46d4eb7bc1d01