models: add inverse_of to WorkshopSponsor belongs_to :workshop

This enables proper Rails association caching when accessing workshop from
a workshop_sponsor record, reducing redundant queries.

Author: mroderick

app/models/workshop_sponsor.rb

@@ -1,6 +1,6 @@
 class WorkshopSponsor < ApplicationRecord
   belongs_to :sponsor
-  belongs_to :workshop
+  belongs_to :workshop, inverse_of: :workshop_sponsors
 
   validates :sponsor_id, uniqueness: { scope: :workshop_id, message: :already_sponsoring }
 end