Add index for projects API default param

This if the default query for the unauthenticated user:

```sql
SELECT
    "projects".*
FROM
    "projects"
WHERE
    "projects"."visibility_level" = 20
    AND "projects"."pending_delete" = FALSE
ORDER BY
    "projects"."created_at" DESC,
    "projects"."id" DESC
LIMIT 20 OFFSET 0
```

We also remove the existing, now redundant index.

Relates to https://gitlab.com/gitlab-org/gitlab/issues/36135

changelogs/unreleased/ab-projects-api-indexes.yml

+---
+title: Add index for unauthenticated requests to projects API default endpoint
+merge_request: 19989
+author:
+type: performance

db/migrate/20191112214305_add_indexes_for_projects_api_default_params.rb

+# frozen_string_literal: true
+
+class AddIndexesForProjectsApiDefaultParams < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :projects, %i(visibility_level created_at id)
+    remove_concurrent_index_by_name :projects, 'index_projects_on_visibility_level'
+  end
+
+  def down
+    add_concurrent_index :projects, :visibility_level
+    remove_concurrent_index :projects, %i(visibility_level created_at id)
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_11_12_115317) do
+ActiveRecord::Schema.define(version: 2019_11_12_214305) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -3145,7 +3145,7 @@ ActiveRecord::Schema.define(version: 2019_11_12_115317) do
     t.index ["runners_token"], name: "index_projects_on_runners_token"
     t.index ["runners_token_encrypted"], name: "index_projects_on_runners_token_encrypted"
     t.index ["star_count"], name: "index_projects_on_star_count"
-    t.index ["visibility_level"], name: "index_projects_on_visibility_level"
+    t.index ["visibility_level", "created_at", "id"], name: "index_projects_on_visibility_level_and_created_at_and_id"
   end
 
   create_table "prometheus_alert_events", force: :cascade do |t|