Enable links beginning with 'www.' even if no protocol is specified

This PR adds a function that fixes standard hyperlinks, which are specified without a protocol (this is a problem in some PDF files). Currently, these kind of links all point to `resource://pdf.js/web/` in Firefox.

I understand that there are security considerations when enabling links, but I do not believe that there should be any real issues if we simply add `http://` only to those links that begin with `www.`.
*If this assessment of the security situation is wrong, please feel free to close this PR.*

Fixes #3741 (and possibly #3284, but I can't check since the file is no longer available).