Upgrade to work in latest Mediawiki

[allcreatures]

A few changes here:

• now works in Mediawiki 1.39 and up
• Added a README based loosely on the old archived Mediawiki page
• Added createpage permission so user with talk permission can create the Discussion page if not already created
• Add some extra checks to ensure the subject page exists before allowing creation of Discussion page

Optionally the updated code also applies to to the users own Talk page and Subject page. You may want to remove this if not something you want. Ideally we might want extra permissions to include: talk, createtalk, profiletalk, createprofiletalk.

The code is updated to suit what a client needed, no more, no less.

Enjoy!