Audit events inaccessibile if event is present where author user was deleted

If an audit event is present in a group/project and a particular user is the 'author' of the event but they have since been completely deleted from GitLab, a 500 error is thrown.

I think we need to make the author information part of the raw details hash. We can opt to try to look up the user at view time but fall back to the raw data, potentially.

Completed 500 Internal Server Error in 69ms (ActiveRecord: 3.8ms) 
ActionView::Template::Error (undefined method `name' for nil:NilClass): 
10: %tbody 
11: - events.each do |event| 
12: %tr 
13: %td #{event.author_name} 
14: %td 
15: %span 
16: #{raw human_text(event.details)} 
app/models/audit_event.rb:31:in `author_name’

cc/ @rdavila @dbalexandre Thoughts?