Doc: replace "user name" with "username"

In the UI and docs, when in the context of
authentication.

Change-Id: I643b5c831ba1d63cf41e8ab31ec98735a2d07dc7
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>