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

.rubocop_todo.yml

@@ -653,15 +653,6 @@ Layout/SpaceInsidePercentLiteralDelimiters:
   Exclude:
     - 'app/models/activity.rb'
 
-# Offense count: 2
-# Cop supports --auto-correct.
-# Configuration parameters: EnforcedStyle, SupportedStyles.
-# SupportedStyles: single_quotes, double_quotes
-Style/StringLiteralsInInterpolation:
-  Exclude:
-    - 'app/controllers/stats_controller.rb'
-    - 'app/models/proposal.rb'
-
 # Offense count: 9
 # Cop supports --auto-correct.
 # Configuration parameters: IgnoredMethods.

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