Remove defaultLayout for join meeting

[samuelwei]

In BBB 3.0 defaultLayout will be removed and replaced by userdata-bbb_default_layout
A new prop enforceLayout will be added (see #216) to enforce a specify layout that cannot be changed by the user.

Setting the default layout on the create call is uneffected by these changes.

https://docs.bigbluebutton.org/3.0/development/api/#updates-to-api-in-bigbluebutton

Questions:

• Should we add BackendEnum Support to userdata?

• How should we proceed with the removal of defaultLayout? Add a deprecation note? Auto add userdata-bbb_default_layout to make the api work with BBB 3.0 and BBB < 3.0