Enhancement - Let a student to participate without leaving and enter the room with another role

[cruizba]

Deployment info

OpenVidu Teaching version: master - 11459a6
Docker Image: openvidu/openviduteaching:1.3.0-beta1
We've built or own docker image by executing ./create_image.sh in https://github.com/codeurjc-students/2019-OpenViduTeaching/tree/master/docker in order to update OpenVidu Client version to 2.15.0.

Issue description:

It would be great to let participants (students) to speak, share video or screen as students, in the way that if anyone rise their hand, you can give to the student permissions to share video and audio without changing the role.