Fix most rubocop issues (#2165)

* Run rubocop --autocorrect on app/, config/ and lib/, also manually fix some remaining style issues

* Run rubocop --autocorrect-all on db/

* Run rubocop --autocorrect-all on `spec/` and fix remaining issues

db/migrate/20171009222537_create_keyword_mutes.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class CreateKeywordMutes < ActiveRecord::Migration[5.1]
   def change
     create_table :keyword_mutes do |t|