group_members_controller.rb

- Disable {project, group} members submit button if no users

  If no users are selected, the submit button should be disabled.

- Alert user when no users were added to {project, group}.

  When no users were selected for adding, an alert message is
  flashed that no users were added.

- Also, this commit adds a feedback when users were actually added to a
  project, in symmetry with how group members are handled.

Closes #22967, #23270.