Add behat test for new UI to manage messaging preferences

Privacy Preferences

Feature

In order to control whether I'm contactable
As a user
I need to be able to update my messaging preferences

Scenarios

Update settings: My contacts only
Update settings: My contacts and anyone in my courses
Update settings: Anyone on the site (Admin enable site-level messaging)

General Preferences

Feature

In order to make message editor more usable
As a user
I need to be able to update my 'Use enter to send' preferences

Scenario

Update 'Use enter to send' preference

Note: Notification preferences will be added in other issue MDL-65221.