Fix broken label uniqueness label migration

The previous implementation of the migration failed on staging because
the migration was attempted to remove labels from projects that did not
actually have duplicates. This occurred because the SQL query did not
account for the project ID when selecting the labels.

To replicate the problem:

1. Disable the uniqueness validation in app/models/label.rb.
2. Create a duplicate label "bug" in project A.
3. Create the same label in project B with label "bug".

The migration will attempt to remove the label in B even if there are no duplicates.

Closes #23609

db/migrate/20161017125927_add_unique_index_to_labels.rb

@@ -7,9 +7,9 @@ class AddUniqueIndexToLabels < ActiveRecord::Migration
   disable_ddl_transaction!
 
   def up
-    select_all('SELECT title, COUNT(id) as cnt FROM labels GROUP BY project_id, title HAVING COUNT(id) > 1').each do |label|
+    select_all('SELECT title, project_id, COUNT(id) as cnt FROM labels GROUP BY project_id, title HAVING COUNT(id) > 1').each do |label|
       label_title = quote_string(label['title'])
-      duplicated_ids = select_all("SELECT id FROM labels WHERE title = '#{label_title}' ORDER BY id ASC").map{ |label| label['id'] }
+      duplicated_ids = select_all("SELECT id FROM labels WHERE project_id = #{label['project_id']} AND title = '#{label_title}' ORDER BY id ASC").map{ |label| label['id'] }
       label_id = duplicated_ids.first
       duplicated_ids.delete(label_id)