Apply Layout/LineLength rubocop rule

Note we're excluding a few files:

* Configuration files that weren't generated by us
* Migration files that weren't generated by us
* The Gemfile, since it includes an important comment that must be on
  the same line as the gem declaration
* The Budget::Stats class, since the heading statistics are a mess and
  having shorter lines would require a lot of refactoring

app/models/poll/booth_assignment.rb

@@ -24,7 +24,9 @@ class Poll
     private
 
       def shifts
-        Poll::Shift.where(booth_id: booth_id, officer_id: officer_assignments.pluck(:officer_id), date: officer_assignments.pluck(:date))
+        Poll::Shift.where(booth_id: booth_id,
+                          officer_id: officer_assignments.pluck(:officer_id),
+                          date: officer_assignments.pluck(:date))
       end
 
       def destroy_poll_shifts