Re-add and apply Rails/UniqBeforePluck rule

We removed it in commit d639cd58 because it recommended using `uniq`
where `distinct` was more appropriate. This has been fixed in
rubocop-rails 2.6.0.

.rubocop.yml

@@ -300,6 +300,9 @@ Rails/SkipsModelValidations:
 Rails/TimeZone:
   Enabled: true
 
+Rails/UniqBeforePluck:
+  Enabled: true
+
 Rails/UniqueValidationWithoutIndex:
   Enabled: true
   Severity: refactor

app/controllers/admin/stats_controller.rb

@@ -1,6 +1,6 @@
 class Admin::StatsController < Admin::BaseController
   def show
-    @event_types = Ahoy::Event.pluck(:name).uniq.sort
+    @event_types = Ahoy::Event.distinct.order(:name).pluck(:name)
 
     @visits    = Visit.count
     @debates   = Debate.with_hidden.count