Make sure settings import doesn't degrade connection security

 Rewrite the now obsolete values "SSL_TLS_OPTIONAL" and "STARTTLS_OPTIONAL"
 to their "*_REQUIRED" counterparts before import.
Remove SimpleX509TrustManager because it's no longer used
Use SSL/TLS as fallback when importing server settings
Enable connection security by default when setting up an account
Remove all entries without transport encryption from providers.xml
Fix typos