Include a default mail server configuration in self-hosted installations

For self-hosted images, the email configuration page is empty. The customer needs to set up the email configuration to be able to Invite users to their Appsmith instance. As this is a critical step to onboard users and greatly improves the onboarding experience for the customer, we should provide a default Appsmith email server to ensure delivery of these emails before the user can set up their custom server.

There is a risk that these emails may end up in spam, but that can be a caveat we can list and encourage users to configure their own SMTP server.

When an external SMTP server is configured, the embedded one, shouldn't run anymore.