The admin fieldset should live in Subscription section

Issue Summary

I believe that the administrative fieldset should be moved on Simplenews admin pages from the 'General' section to the 'Subscription' part.

'General' is described as Simplenews content type and vocabulary settings and reports a vocabulary update on each submit, so our settings are not really fitting there.

'Subscription' is described as Subscription settings, opt-in/out confirmation email text, which is much closer, and makes me always look for the simplenews_register settings in there. Also there's a user profiles related setting already (the 'Synchronize with account' checkbox).

To my eye, this is minor, yet very obvious mismatch on the UI.