Change the UI for Participants to User/Role Mapping

Users constantly report:

Removing the "All users enrolled" BBB role mapping does not prevent users from joining the BBB meeting.

This is due a misconception of the feature as the isea is to have a mapping for Moodle Users/Roles with BigBlueButton Roles.

https://github.com/blindsidenetworks/moodle-mod_bigbluebuttonbn/issues/244