Implement presets at room creation

[erikjohnston]

Adds the ability to specify a preset when creating a room. Currently defined are:

• public_chat - for completely public rooms, where anyone can join and everyone can see history.
• private_chat - for 1:1 and private group chats, invite only and people can only see history after they were invited.

[kegsay]

Would really prefer CONSTANT_NAMES here :)

[kegsay]

LGTM other than in-line comments. I'm a little hesitant to use private_chat to mean "invite only and people can only see history after they were invited". What would we call rooms which you had to knock on? Or ones with a password? Are they not private? We may want to be more explicit here to avoid us having awkward situations in the future. Maybe private_invite_chat, private_knock_chat and private_password_chat? Not sure.

[erikjohnston]

What would we call rooms which you had to knock on?

The idea behind these presets is to encourage apps to use these defaults when a user wants to create a room for a given purpose, giving a simple and consistent UX across apps. I.e., these presets are named after what the user wanted, rather than describing the actual defaults.

If the user/app wants to do something more complex they wouldn't use a preset, but would instead specify the options they want (though I haven't actually implemented that part of the API yet).