Add #metadata to BigbluebuttonRoom, just as in BigbluebuttonRecording
Add creation, edition and deletion of metadata in room's views

Metadata are edited as nested attributes
Metadata names can't have underlines

This is way: https://github.com/bigbluebutton/bigbluebutton/blob/master/bigbluebutton-web/src/java/org/bigbluebutton/api/ParamsProcessorUtil.java#L291