UsersGroupsRightsManagementIT#createAndDeleteUser is flickering

The test clicks on the "Create user" button and then waits for the Create User modal to appear. The problem is that the code that loads the content of the Create User model can register its show.bs.modal after the "Create user" is clicked, if the test is fast enough. Note that the test does wait for the users live table to load, but still this can happen before the Create User modal is enhanced.

The solution is to generate the "Create user" button as disabled from the server and to enable it from JavaScript only after the Create User modal is ready. This way the test can wait for the "Create user" to be enabled (besides waiting for the live table).