Upgrade Doorkeeper from 3.1.0 to 4.0.0.
Includes Rails 5 support and various bug fixes. Working toward #14286 (moved).
Changelog: https://github.com/doorkeeper-gem/doorkeeper/blob/master/NEWS.md#400
cc: @rspeicher
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Includes Rails 5 support and various bug fixes. Working toward #14286 (moved).
Changelog: https://github.com/doorkeeper-gem/doorkeeper/blob/master/NEWS.md#400
cc: @rspeicher