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

app/serializers/rest/mute_serializer.rb

@@ -2,7 +2,7 @@
 
 class REST::MuteSerializer < ActiveModel::Serializer
   include RoutingHelper
-  
+
   attributes :id, :account, :target_account, :created_at, :hide_notifications
 
   def account
@@ -12,4 +12,4 @@ class REST::MuteSerializer < ActiveModel::Serializer
   def target_account
     REST::AccountSerializer.new(object.target_account)
   end
-end
+end