Move events index datatable row to a partial

- clean up columns
- tag unregistered submitters & speakers
- implement caching

app/models/event_user.rb

@@ -2,6 +2,6 @@ class EventUser < ApplicationRecord
   # TODO: Do we need these roles?
   ROLES = [%w[Speaker speaker], %w[Submitter submitter], %w[Moderator moderator]]
 
-  belongs_to :event
+  belongs_to :event, touch: true
   belongs_to :user
 end