Assure screen object is set.

https://wordpress.org/support/topic/php-non-object-notice#post-6055863
https://wordpress.org/support/topic/php-non-object-notice-1#post-6055880
Don't show admin notice for registration setting

if S2Member plugin is active
Add comments.
Add user meta when user logs in.
Add a comment to the login_user function.
If user has signed in, don't reset password.
Update readme with bug fix details.
Update changelog for 1.7.1
Bump plugin version to 1.7.1
Update translation files.