Enable Style/DoubleNegation Rubocop cop

This cop checks for uses of double negation (!!) to convert something to a boolean value. It doesn't support autocorrection, so all the offenses where manually solved.

Closes #1374

.rubocop.yml

@@ -122,6 +122,10 @@ Style/TrailingBlankLines:
 Style/TrailingWhitespace:
   Enabled: true
 
+# Checks for uses of double negation (!!) to convert something to a boolean value.
+Style/DoubleNegation:
+  Enabled: true
+
 AllCops:
   Include:
     - '**/Rakefile'

.rubocop_todo.yml

@@ -246,11 +246,6 @@ Style/Documentation:
 Style/DotPosition:
   Enabled: false
 
-# Offense count: 5
-Style/DoubleNegation:
-  Exclude:
-    - 'app/models/conference.rb'
-
 # Offense count: 1
 # Cop supports --auto-correct.
 Style/ElseAlignment:

app/models/conference.rb

@@ -838,7 +838,7 @@ class Conference < ActiveRecord::Base
   # * +True+ -> If conference has a venue object.
   # * +False+ -> IF conference has no venue object.
   def venue_set?
-    !!venue
+    venue.present?
   end
 
   ##
@@ -848,7 +848,7 @@ class Conference < ActiveRecord::Base
   # * +True+ -> If conference has a cfp object.
   # * +False+ -> If conference has no cfp object.
   def cfp_set?
-    !!program.cfp
+    program.cfp.present?
   end
 
   ##
@@ -858,7 +858,7 @@ class Conference < ActiveRecord::Base
   # * +True+ -> If conference has a start and a end date.
   # * +False+ -> If conference has no start or end date.
   def registration_date_set?
-    !!registration_period && !!registration_period.start_date && !!registration_period.end_date
+    registration_period.present? && registration_period.start_date.present? && registration_period.end_date.present?
   end
 
   # Calculates the distribution from events.