If `<!doctype html>` is not found in static/email-templates/base.html,
all system e-mail templates are assumed to be plaintext and go out
as content-type: plaintext e-mails. With this, all HTML tags can
be stripped out of the system e-mail templates (while maintaining
Go template tags and logic) to have plaintext system e-mail templates.
Closes#546
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.
