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.
This reverts commit fb70dc8c99, reversing
changes made to 53fef63a9a.
Conflicts:
pkg/fedora/diaspora-setup
pkg/ubuntu/diaspora-setup
public/stylesheets/sass/application.sass
spec/models/user/attack_vectors_spec.rb
