Previously send_email was part of CloverWeb. This makes it difficult to send an
email in contexts outside of the web application. Sending mail requires setting
up a whole web app, which is not a lightweight operation. The reason of putting
send_email in CloverWeb was to have access to render plugin, because we need to
render html pages as a part of email content. render plugin is part of roda, so
it is not available outside of roda app.
In this commit, I implemented a simple EmailRenderer. It is a wrapper around
Tilt::ErubiTemplate, which is the same as what is used in roda's render plugin.
With the new renderer, we can render templates without having to set up a roda
app and send emails from any context.
This also unblocks sending monthly emails for invoices.
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