fix bug 753563 - Support username changes

Users now find an extra field on their profile edit page titled `Username`. Changing this
field allows them to change their username.
Existing usernames that do not comply with the new validation rules will not be affected.
I would suggest we consider the following formatting, which may be more readable. 

<blockquote>User name may contain only letters, numbers, and these characters: . - _</blockquote>

This is the copy Janet, our copy editor, <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=866533#c7">has recommended in the past</a>.