Allow to append custom parameters on create and join

BigBlueButton can work in many different ways depending of the parameters that are passed to it on Create and Join. This allows many other systems, load balancer and external applications to smoothly integrate with BBB and make use of the recordings on meetings created through Moodle.

The general cases are already covered through settings, but it would be hard to add settings (Plugin or Instance) for all of them. The idea is to add a general box on both, Plugin settings and Instance settings, so Administrators can append global settings either on create or Join, and even Teachers can include those on certain rooms.