paginate and sort candidates list

comletes #99

app/controllers/admin/candidate_controller.rb

@@ -3,8 +3,11 @@ module Admin
   class CandidateController < AdminController
     before_action :collect_quizzes, except: [:login, :auth]
 
+    helper_method :sort_column
+
     def index
-      @candidates = policy_scope Candidate.order(:name)
+      @candidates = policy_scope Candidate.order("#{sort_column} #{sort_direction}")
+                    .page(params[:page])
     end
 
     def new
@@ -65,5 +68,9 @@ module Admin
         CandidateMailer.welcome(candidate).deliver_later
         RecruiterMailer.candidate_created(candidate).deliver_later
       end
+
+      def sort_column
+        Candidate.column_names.include?(params[:sort]) ? params[:sort] : 'name'
+      end
   end
 end

app/controllers/admin/result_controller.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 module Admin
   class ResultController < AdminController
-    helper_method :sort_column, :sort_direction
+    helper_method :sort_column
 
     # TODO: change context from Candidate to Quiz
     # bypass pundit lockdowns until completed
@@ -30,9 +30,5 @@ module Admin
       def sort_column
         Candidate.column_names.include?(params[:sort]) ? params[:sort] : 'completed_at'
       end
-
-      def sort_direction
-        %w(asc desc).include?(params[:direction]) ? params[:direction] : 'desc'
-      end
   end
 end