Add ability to send "System messages" via DMs

[taoeffect]

Problem

Sometimes we want to be able to inform users of important upcoming changes.

For example, for 2.0, we're going to be forced to wipe all data on the server one final time, and we'd like to give users an in-app heads up about this.

Likewise, each time a new version is released, we want users to see the release notes, and any accompanying blog.

Solution

1. Create a DM to the user from Group Income (with the GI logo) if one doesn't exist.
2. In this chatroom contract, have it maintain a state (as a simple object, e.g. "systemMessages") of system messages to keep track of which ones we've already sent (to avoid sending duplicates).
3. Check if we haven't yet sent this system message using the chatroom state mentioned in (2). If we haven't, then send the message. Create a special action on the chatroom contract that both posts the message and updates the chatroom state "systemMessages" as having sent that particular system message.

Example "systemMessages":

"systemMessages": {
  "2.0update-warning": true // indicates we've already sent this DM
}

[snowteamer]

What are the pros and cons of sending such system messages via DMs rather than via notifications?

[taoeffect]

@snowteamer Sending it via notifications was my initial thought, but then I decided against it. I don't remember exactly why, but one thing I can think of now is that in-app notifications aren't setup to handle large amounts of text or markdown formatting.