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


Somewhat inspired by our old layer and somewhat to abstract. If, for example, we changed repositories to have their own permissions, the only thing we have to change is what the guts of Repository.readable does. Further, if you look at objects that do not have their own permissions, such as repositories, you'd see that they can't simply call `authorized` -- https://github.com/Katello/katello/blob/master/app/models/katello/authorization/repository.rb#L36-L38