WEB-18 Conference with at least two participants

changing workflow of connection establishment. a new participant needs to set up its video stream, after that a 'hello' is send to the server which is followed by connection OFFERs of all participants. these offers will be ANSWERed by the new participant and its stream is added
-> tested and worked with at least two participants
-> solves WEB-15 & WEB-19

design adjusted