Fix all Style/StringLiteralsInInterpolation rubocop issues and remove from rubocop_todo list

app/controllers/stats_controller.rb

@@ -23,6 +23,6 @@ class StatsController < ApplicationController
   private
 
     def daily_cache(key, &block)
-      Rails.cache.fetch("public_stats/#{Time.current.strftime("%Y-%m-%d")}/#{key}", &block)
+      Rails.cache.fetch("public_stats/#{Time.current.strftime('%Y-%m-%d')}/#{key}", &block)
     end
 end

app/models/proposal.rb

@@ -128,7 +128,7 @@ class Proposal < ActiveRecord::Base
   end
 
   def code
-    "#{Setting["proposal_code_prefix"]}-#{created_at.strftime('%Y-%m')}-#{id}"
+    "#{Setting['proposal_code_prefix']}-#{created_at.strftime('%Y-%m')}-#{id}"
   end
 
   def after_commented