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:

1. The Mailer is disabled, but email activations are required
2. 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)