Emit proper HTML document from web service and make image src relative

* Emit a proper HTML document including html, head and body elements. The head
  element contains a base href pointing to the wiki's article URL so that our
  relative links resolve when viewed directly from the web service.

* Make image src attributes relative. This should eliminate all absolute links
  pointing to the wiki itself.

* Store the wiki's article base URI in conf.wiki.baseURI.

No change in parser tests.

Change-Id: I8c191cfc874f8ed7d8d22dace8812005237269c8