Add and apply Rails/SelectMap rule

This rule was introduced in rubocop-rails 2.21.0. Using `pluck` is
easier to read.

.rubocop.yml

@@ -460,6 +460,9 @@ Rails/SaveBang:
   Enabled: true
   Severity: refactor
 
+Rails/SelectMap:
+  Enabled: true
+
 Rails/SkipsModelValidations:
   Enabled: true
   ForbiddenMethods:

app/controllers/admin/poll/officer_assignments_controller.rb

@@ -7,7 +7,7 @@ class Admin::Poll::OfficerAssignmentsController < Admin::Poll::BaseController
     @officers = ::Poll::Officer
                 .includes(:user)
                 .order("users.username")
-                .where(id: @poll.officer_assignments.select(:officer_id).distinct.map(&:officer_id))
+                .where(id: @poll.officer_assignments.distinct.pluck(:officer_id))
                 .page(params[:page])
                 .per(50)
   end