Makes some static source code analyzers happier.

Killed the old static route to change a users display name and moved it
to a properly testable controller.

Steps to reproduce:
* having an unknown language set in oc_preferences
* browse the personal settings
* only get listed the first letter of this language in the language chooser

* introduced with #20135 - c6f6a875

Fixes following error meessage:

{"reqId":"GliKwPFSIVn8GC2KNIHY","remoteAddr":"::1","app":"PHP","message":"Undefined variable: subAdmins at master\/settings\/users.php#76","level":0,"time":"2015-11-04T08:38:53+00:00","method":"GET","url":"\/master\/index.php\/settings\/users"}
{"reqId":"GliKwPFSIVn8GC2KNIHY","remoteAddr":"::1","app":"PHP","message":"Invalid argument supplied for foreach() at master\/settings\/users.php#76","level":0,"time":"2015-11-04T08:38:53+00:00","method":"GET","url":"\/master\/index.php\/settings\/users"}

Also moved the logic to the UsersController