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.
What sort of "known application" did you have in mind?  I picture users adding OAuth applications to their Huginn instances when they want to authorize something... otherwise wouldn't everyone know all the secret keys?