Merge branch 'rubocop/disable-uniq-before-pluck-cop' into 'master'

Disable Rails/UniqBeforePluck rubocop cop

 Rails/UniqBeforePluck seems to have some bugs 

* https://github.com/bbatsov/rubocop/issues/3122
* https://github.com/bbatsov/rubocop/issues/3148

and we had some problems in EE with that https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/425#note_12245005

See merge request !4477

.rubocop.yml

@@ -1088,6 +1088,9 @@ Rails/TimeZone:
 Rails/Validation:
   Enabled: false
 
+Rails/UniqBeforePluck:
+  Enabled: false
+
 ##################### RSpec ##################################
 
 # Check that instances are not being stubbed globally.