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.
My idea was to create known applications via migrations. So normal huginn users (and admins) do not have to create them on their own. The applications secret needs to be public anyways, otherwise you would need to copy a generated secret from the application to huginn before you could use it.