Correctly construct the ContentEntityForm object

This is a fix for Drupal 8.3.x

WorkspaceForm extends the ContentEntityForm but it doesn't construct it correctly. This proces errors like this:

Error: Call to a member function getRequestTime() on null in Drupal\Core\Entity\ContentEntityForm->updateChangedTime() (line 373 of /var/www/drupal8dev/core/lib/Drupal/Core/Entity/ContentEntityForm.php) #0 /var/www/drupal8dev/core/lib/Drupal/Core/Entity/ContentEntityForm.php(139): Drupal\Core\Entity\ContentEntityForm->updateChangedTime(Object(Drupal\multiversion\Entity\Workspace)) #1 [internal function]: Drupal\Core\Entity\ContentEntityForm->submitForm(Array, Object(Drupal\Core\Form\FormState)) #2 /var/www/drupal8dev/core/lib/Drupal/Core/Form/FormSubmitter.php(111): call_user_func_array(Array, Array) #3 /var/www/drupal8dev/core/lib/Drupal/Core/Form/FormSubmitter.php(51): Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object(Drupal\Core\Form\FormState)) #4 /var/www/drupal8dev/core/lib/Drupal/Core/Form/FormBuilder.php(585): Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object(Drupal\Core\Form\FormState)) ...