Move or update Gatekeeper

Howdy :-) I worked on this project back in 2015 (when Corona was just a tasty beverage) and I thought I handed over all related accounts to the Ember CLI org but it turns out I missed one :-)

[Gatekeeper](https://github.com/prose/gatekeeper) (the tiny server that facilitates the oauth signing into Github) runs on a very, very, very old Heroku instance which turned out to be registered to a gmail account of mine :-)

This instance could most likely also be run as a simple lambda function / serverless app in the AWS environment where the rest of the backend also runs. It might make most sense to simply set it up there and point to the new location from the config. Otherwise, I can also hand over the Heroku account.