MB-4643: maxlength attribute for login form fields

Change-Id: Ibd59a42281ca5686804ca99ee6ad93ac50c7251a
Reviewed-on: http://review.couchbase.org/30231
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>