Add prompt when deleting multiple users

[olorton]

Deleting a single user has a prompt to confirm that the action should be carried out. When deleting multiple users this does not happen.

To reproduce:

1. Navigate to the Admin User page
2. Select an admin via the checkbox (to the left of their username)
3. Click on the 'Select an action' drop down box and select the delete option
4. Hit the 'OK' button

Result
The user is successfully deleted but there is no confirmation prompt message which states to the admin that they are about to delete a user
The message "The elements were processed successfully" is displayed to the user instead, which doesn't seem very user friendly
Please see the screenshot
Raised against BOARD-56