seo: fix canonical tags on post index and show page.

* 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.

test/test_helper.rb

@@ -64,6 +64,7 @@ class ActionDispatch::IntegrationTest
   extend ControllerHelper
 
   register_encoder :xml, response_parser: ->(body) { Nokogiri.XML(body) }
+  register_encoder :html, response_parser: ->(body) { Nokogiri.HTML5(body) }
 
   def method_authenticated(method_name, url, user, **options)
     post session_path, params: { name: user.name, password: user.password }