Changed the web conference permissions in spaces

Formerly the permissions of the spaces were as follows:

 - All members of a space will be moderators in its web conference room;
 - All members of a space can start a web conference in the space;
 - All members of a space can stop a web conference in the space.

Now this is the new set of permissions:

 - Only administrators of a space and the person that started the web conference will be moderators;
 - All members of a space can start a web conference in the space;
 - Only administrators of a space and the person that started the web conference can stop the conference in the space.

Required the new version of the bigbluebutton_rails gem.

Refs #1148