Allow the configuration of a welcome message at the site level

Welcome message can be set up per activity and it it is open for Teachers to edit it. But, there are cases when the institution wants to set up a fixed welcome message for all the meetings or just to use the one set up in BBB with no changes. 

In order to cover those cases, it would be required to allow Welcome messages at the global level, and add the option for enable/disable the item from the Activity settings, and even to controll it through the config.php variables (as most of the other features).