Make AdminLinks installable via Composer
- Update global vars in AdminLinks.php to use $GLOBALS...
- Create composer.json
- Bump version number in README
- Used enterprisemediawiki/admin-links for intial Composer testing
with packagist.org, but switched to mediawiki/admin-links for commit.
This will not work until AdminLinks is added to MediaWiki's Packagist
account, but that can't happen until there is a functioning composer
commit in Gerrit.