Custom room types

[mrsimpson]

When enhancing Rocket.Chat, there are multiple levels. While Rocket.Chat Apps are meant to be enhancing the functional scope ("what you can do in the context of a conversation"), there are use cases in which specific behavior for particular types of conversations is needed.
In Rocket.Chat vocabulary, this means the introduction of new room types.

Fortunately, there is already an API for adding new room-types. But on the downside, just getting a room type registered does not mean that it's possible to interact with it: One needs to be able to create and find those rooms. Currently, this means modifications to the Blaze-templates. Finally, there are a couple of places where the in-built-room types are coded.
This issue is about enabling the RoomTypes-API so that room types can be added without modification of RC core.

Aim is that custom functionality based on room types is possible by providing an own meteor package which is added at build-time based on the un-modified, released version of Rocket.Chat core.

[mrsimpson]

@RocketChatApp I'd be willing to contribute that development. Let me know in advance whether you are willing to accept a PR with this contents.
I'd start with sketching the changes in a PR right away. I'll keep it as simple as possible (no architectural candy) and in such a way that you can see that it's 100% compatible.