Add tracking categories when magic links are used

This adds 3 tracking categories, one for each type of magic link (ISBN,
RFC, PMID). This will allow wikis to gauge usage and identify pages that
need migrating.

These will only show up if the respective magic links are enabled via
$wgEnableMagicLinks.

Change-Id: Ic483f0c493112bf6373e1b37961e1241c20c3582