Notify users about the echo test setting the first time

[tibroc]

What?

When #461 is implemented, it would be nice to show users that this options exists and explain it to them. This could be done by having a pop-up menu that only opens the first time a user enters a meeting.

How?

When a user enters a meeting for the first time, they see a pop-up that explains to them the echo-test option:

If the user clicks on “Deactivate audio testing” the setting should be changed to deactivated and then join the BBB session with the updated settings.
This pop-up should only appear if the default set by the plugin-admin is to have the echo-test.