Drop halt_callback_chains_on_return_false

ActiveSupport.halt_callback_chains_on_return_false= is deprecated and
will be removed in Rails 5.2. Stop using it and return `throw(:abort)`
instead in the callbacks which need it.

app/models/conference.rb

@@ -1120,7 +1120,6 @@ class Conference < ApplicationRecord
   #
   def create_email_settings
     build_email_settings
-    true
   end
 
   ##

app/models/program.rb

@@ -211,7 +211,6 @@ class Program < ApplicationRecord
     EventType.create(title: 'Workshop', length: 60, color: '#0000FF', description: 'Interactive hands-on practice',
                      minimum_abstract_length: 0,
                      maximum_abstract_length: 500, program_id: id)
-    true
   end
 
   ##
@@ -227,7 +226,6 @@ class Program < ApplicationRecord
     DifficultyLevel.create(title: 'Hard',
                            description: 'Events require expert knowledge of the topic.',
                            color: '#EF6E69', program_id: id)
-    true
   end
 
   ##

app/models/role.rb

@@ -18,6 +18,6 @@ class Role < ApplicationRecord
 
   # Needed to ensure that removing all user from role doesn't remove role.
   def cancel
-    false
+    throw(:abort)
   end
 end