Allow apps to create app user's access token

[cuonghuunguyen]

Currently, if we want the app to call API requests, we have to create a new user (only for that app) and provide the credential to the app. I think it is an ugly way to let the app access more complicated requests from Rocket.Chat. Because the app user is generated along with the app and used by the app, I think we can let the apps generate its own app user's access token. We can restrict the app's permission by using the Rocket.Chat built-in user permission system. Also, it can help us to use the extra functions without having to build bridges. What do you think about this idea? @d-gubert @lolimay