This was a ridiculous miss, where on first time installation, the
well designed default e-mail template was never installed in the
DB! I never spotted this because my local dev setup, and
surprisingly, nobody ever complained that the default campaign
template was a blank slate with no styles.
This is a major feature that builds upon the `Messenger` interface
that has been in listmonk since its inception (with SMTP as the only
messenger). This commit introduces a new Messenger implementation, an
HTTP "postback", that can post campaign messages as a standard JSON
payload to arbitrary HTTP servers. These servers can in turn push them
to FCM, SMS, or any or any such upstream, enabling listmonk to be a
generic campaign messenger for any type of communication, not just
e-mails.
Postback HTTP endpoints can be defined in settings and they can be
selected on campaigns.
Kailash Nadh