* Fix incorrect canonical tags. Before we were using
`<meta name="canonical" content="...">`. This is wrong, it should have been
`<link rel="canonical" href="...">`.
* Add a default canonical tag on all pages. Fixes Google treating the
same content on different subdomains (safebooru, shima, kagamihara, etc)
as duplicate content. Also fixes Google sometimes treating similar but
distinct content on the same domain as duplicate content.