Don't let users configure invalid email settings
The following combination of settings are invalid, and AdminFE should warn or possibly even prevent users from setting them:
- The Mailer is disabled, but email activations are required
- The Mailer is disabled, but the Welcome Email is enabled
In either case, the user should be instructed to either revert the setting, or enable & configure the Mailer.
Related: pleroma#2606 (closed)