Add option to disable username changing

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.
Merge pull request #2991 from raphendyr/username_change_disablation

Add option to disable username changing