fixes #7668 - errata summary mail

Don't merge yet, needs https://github.com/theforeman/foreman/pull/1813.


Here's how it looks (HTML and text): 
   https://gist.github.com/stbenjam/2961563b88eab1093b00


Could do, but is this permament? I'd rather call the methods from Foreman's `Authorizable` mixin directly... I don't really understand our layer in the middle, I assume because of Katello's old system?