Add index to labels for `type` and project_id`

When loading pages that display the number of open issues,
the backend runs a query such as:

```sql
SELECT "labels"."id" FROM "labels" WHERE "labels"."type" IN ('ProjectLabel') AND "labels"."project_id" = 1000
```

This results in an entire scan of the `labels` table. To optimize performance,
add the appropriate index to the table.

Closes #27676

db/migrate/20170204181513_add_index_to_labels_for_type_and_project.rb

+class AddIndexToLabelsForTypeAndProject < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def change
+    add_concurrent_index :labels, [:type, :project_id]
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170130204620) do
+ActiveRecord::Schema.define(version: 20170204181513) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -576,6 +576,7 @@ ActiveRecord::Schema.define(version: 20170130204620) do
   end
 
   add_index "labels", ["group_id", "project_id", "title"], name: "index_labels_on_group_id_and_project_id_and_title", unique: true, using: :btree
+  add_index "labels", ["type", "project_id"], name: "index_labels_on_type_and_project_id", using: :btree
 
   create_table "lfs_objects", force: :cascade do |t|
     t.string "oid", null: false