This commit removes the Go html2text lib that would automatically
convert all HTML messages to plaintext and add them as the alt
text body to outgoing e-mails. This lib also had memory leak
issues with certain kinds of HTML templates.
A new UI field for optionally adding an alt plaintext body to
a campaign is added. On enabling, it converts the HTML message in
the campaign editor into plaintext (using the textversionjs lib).
This introduces breaking changes in the campaigns table schema,
model, and template compilation.
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.
Certain SMTP hosts limit the total number of messages that can be
sent within a window, for instance, X / 24 hours. The concurrency
and message rate controls can only limit that to a max of
1 messages / second, without a global cap.
This commit introduces a simple sliding window rate limit feature
that counts the number of messages sent in a specific window, and
upon reaching that limit, waits for the window to reset before
any more messages are pushed out globally across any number of
campaigns.
Context: https://github.com/knadh/listmonk/issues/119
