limit candidate quiz dropdown to only show "active" quizzes

completes #105

app/models/quiz.rb

@@ -8,4 +8,10 @@ class Quiz < ApplicationRecord
   validates :name, presence: true, uniqueness: true
   validates :dept, presence: true
   validates :unit, presence: true
+
+  class << self
+    def all_active
+      Quiz.joins(:questions).distinct
+    end
+  end
 end