Implement forum topic voting and tag change pruning (#3580)

db/migrate/20180413224239_create_forum_post_votes.rb

@@ -0,0 +1,14 @@
+class CreateForumPostVotes < ActiveRecord::Migration[5.1]
+  def change
+    create_table :forum_post_votes do |t|
+      t.integer :forum_post_id, null: false
+      t.integer :creator_id, null: false
+      t.integer :score, null: false
+
+      t.timestamps
+    end
+
+    add_index :forum_post_votes, :forum_post_id
+    add_index :forum_post_votes, [:forum_post_id, :creator_id], unique: true
+  end
+end