Respect Accounts_AllowUsernameChange

[m1-key]

• Your Rocket.Chat Experimental app version: 4.2.1.999999999
• Your Rocket.Chat server version: 2.5.0-develop open.rocket.chat
• Device (or Simulator) you're running with: Android 9

Username once set at the time of registration should be non-editable as it will ensure that one user have one username. As currently one user can set different usernames. A warning message should pop up while confirming the username that it is non-editable in future.
Is it a relevant issue/feature ??

[diegolmello]

There's a setting for that on admin:

You can create a PR to check for that setting.
Thanks.