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


Ok, I changed the `-able` methods then to take optional user arguments - tests will explode until the other PR is merged with the `authorized_as` functionality in foreman.