markerb is dead and isn't compatible with rails 6 anymore. Removing it
means adding separate templates for text and html, this sadly means some
code duplication, but also luckily the templates aren't that
complicated, so the duplication is also very minimal.
* Add markerb to the Gemfile
* Update mailer classes to send multipart text+html emails
* Simplify all email templates and convert to Markdown+ERB
This required stripping the CSS and images from the emails. All
Markdown templates are based on the original HTML version except
for the inviter, where it was much easier to work with the text
version (and the i18n contains the whole message anyway).
