Adds doorkeeper to allow application to authenticate against huginn

This should help with #626, it also has the benefit that users do not need to share their username and password with any third party application that needs to access the huginn instance.

Since we probably can not keep the application `secret` secret I think we should automatically add every known application that is able to use huginn via a migration.

At the moment you need to access `<your huginn instance>/oauth/applications` and add the application manually.
I was always thought the secret was just used to verify the application was really the application you expected to authenticate against and the user authentication itself would be a separate part, but I have to read up on that. If that isn't the case I think we should go with a token based approach.