Automatically activate users whose email matches specific email patterns defined in settings (ASTAKOS_RE_USER_EMAIL_PATTERNS) &  minor other fixes:

* introduce email_verified AstakosUser field and if False during signup send email verification
* show recaptcha fields in signup form in case of invited user
* do not allow multiple invitations with the same receiver
* enable user level modification by ``snf-manage modifyuser``

Refs: #2166