Rewrite related tags implementation.

Rewrite the implementation of related tags to be simpler, faster, and
more accurate:

* The related tags are now calculated by taking a random sample of 1000
  posts, finding the top 250 most frequent tags among those posts, then
  ordering those tags by cosine similarity.

* Related tags can generally be calculated in 50-300ms at these sample
  sizes. Very high sample sizes (25000+ posts) are still relatively fast
  (1-3 seconds), but generally they don't improve accuracy much.

* Related tags are now cached in redis rather than in the tags table.
  The related_tags column in the tags table is no longer used.

* Only the related tags in the search taglist are cached. The related
  tags returned by the 'Related tags' button are not cached.

* The cache lifetime is a fixed 4 hours.

* The 'Related tags' button now works with metatags.

* The /related_tag page now works with metatags and multitag searches.

Fixes #4134, #4146.

test/functional/related_tags_controller_test.rb

@@ -2,11 +2,20 @@ require 'test_helper'
 
 class RelatedTagsControllerTest < ActionDispatch::IntegrationTest
   context "The related tags controller" do
+    setup do
+      create(:post, tag_string: "touhou")
+    end
+
     context "show action" do
       should "work" do
         get related_tag_path, params: { query: "touhou" }
         assert_response :success
       end
+
+      should "work for .json responses" do
+        get related_tag_path(format: :json), params: { query: "touhou" }
+        assert_response :success
+      end
     end
   end
 end