Member settings is not being saved

Problem/Motivation

After #3165291: Make the profile configuration form more organized was committed, it is not possible to save the member configuration form.

Steps to reproduce

1) Edit a member configuration form.
2) Add a '*' to the 'Allowed file extensions' field.
3) Save it.
4) Open the form to edit a member configuration again.

You will see that the field 'Allowed file extensions' is blank.

Proposed resolution

Add the #parents attribute to the fields.