This option allows to disable users from changing their username.

This is very usefull in environments using strong internal authentication methods like ldap, pam or shibboleth.

You can allow users to change theyr username in these environments, but then new users (users loging in first time) is blocked from gitlab is her username exists.

Refactored profile to resource. Added missing flash notice on successfull updated. Update username via ajax

TYPO #1870