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/post_migrate/20180813160548_post_migrate_filters.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class PostMigrateFilters < ActiveRecord::Migration[5.2]
   disable_ddl_transaction!
 
@@ -5,7 +7,5 @@ class PostMigrateFilters < ActiveRecord::Migration[5.2]
     drop_table :glitch_keyword_mutes if table_exists? :glitch_keyword_mutes
   end
 
-  def down
-  end
+  def down; end
 end
-