PERF: add index on user auth tokens (for user lookups)

This speeds up user destroy cause we cascade looking up all the tokens
for a user

db/migrate/20190426123658_add_index_on_user_auth_token_user.rb

@@ -0,0 +1,5 @@
+class AddIndexOnUserAuthTokenUser < ActiveRecord::Migration[5.2]
+  def change
+    add_index :user_auth_tokens, [:user_id]
+  end
+end