Merge branch 'upgrade-doorkeeper' into 'master'

Upgrade Doorkeeper from 2.2.2 to 3.1.0

I’d rather upgrade one major version at a time, so Doorkeeper needs to
be upgraded to 3.x before it can be upgraded to 4.x (which includes
Rails 5 support).

Changelog:
https://github.com/doorkeeper-gem/doorkeeper/blob/master/NEWS.md#310

Working toward #14286.

See merge request !3644