Set up /settings/keyword_mutes. #164.

This should eventually be accessible via the API and the web frontend,
but I find it easier to set up an editing interface using Rails
templates and the like.  We can always take it out if it turns out we
don't need it.

spec/controllers/settings/keyword_mutes_controller_spec.rb

@@ -0,0 +1,5 @@
+require 'rails_helper'
+
+RSpec.describe Settings::KeywordMutesController, type: :controller do
+
+end