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.
68afd61024