Allow enabling pair programming study through env

lib/pair_programming23_study.rb

@@ -1,8 +1,10 @@
 # frozen_string_literal: true
 
 class PairProgramming23Study
+  ENABLE = ENV.fetch('PAIR_PROGRAMMING_23_STUDY', nil) == 'true'
+
   def self.participate?
     # TODO: Decide which users are in the study
-    false
+    ENABLE
   end
 end