Use mb_* string methods to extract first character for generated avatars

This fixes #8451 where the first character is a non-ASCII character. The `$string[0]` notation only extracted one byte and thus resulting in an invalid code. The `mb_strtoupper` method also allows to convert characters independently from the current locale on the server.

See also http://php.net/manual/en/function.mb-strtoupper.php



Signed-off-by: Morris Jobke <hey@morrisjobke.de>