* In the bigbluebutton_proxy::action_url call to submodules, we need a way to get
the instance this is called upon so we can have information (parameters) for
this instance settings (like the new settings introduced by subplugins).
* Add the optional instanceid parameter in join/create/getMeetingInfo calls
* Add a new way to extend the BigBlueButtonBN form via subplugins
* Allow subplugins to extends information via new tables and use it in
the edit form.
* Add a new cache helping with potential perfomance issues introduced
by recurring calls to find out about associated tables in subplugins.
Laurent David