Upgrade Rails to version 5.2.1 and adopt code & specs where necessary

Signed-off-by: Sebastian Serth <Sebastian.Serth@student.hpi.de>

spec/controllers/external_users_controller_spec.rb

@@ -14,7 +14,7 @@ describe ExternalUsersController do
   end
 
   describe 'GET #show' do
-    before(:each) { get :show, id: users.first.id }
+    before(:each) { get :show, params: { id: users.first.id } }
 
     expect_assigns(user: ExternalUser)
     expect_status(200)