Add project ID index to `project_authorizations` table to optimize queries

As described in #27443, the `project_authorizations` table is often used to retrieve
all team members of this project. This can lead to a number of slow queries impacting
load times. This MR adds an index for just `project_id`.

Closes #27443

changelogs/unreleased/sh-add-project-id-index-project-authorizations.yml

+---
+title: Add project ID index to `project_authorizations` table to optimize queries
+merge_request:
+author:

db/migrate/20170130204620_add_index_to_project_authorizations.rb

+class AddIndexToProjectAuthorizations < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index(:project_authorizations, :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: 20170121130655) do
+ActiveRecord::Schema.define(version: 20170130204620) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -874,6 +874,7 @@ ActiveRecord::Schema.define(version: 20170121130655) do
     t.integer "access_level"
   end
 
+  add_index "project_authorizations", ["project_id"], name: "index_project_authorizations_on_project_id", using: :btree
   add_index "project_authorizations", ["user_id", "project_id", "access_level"], name: "index_project_authorizations_on_user_id_project_id_access_level", unique: true, using: :btree
 
   create_table "project_features", force: :cascade do |t|