Implement direct messages

[andywynkoop]

Implement a direct messages interface to mimic channels:

• At the db level, direct messages will be channels
• Update channel model to include a boolean to indicate if it is a direct message or channel
• Refactor channel create modal to work for direct messages as well
• Messages will be fetched when workspace loads (because they're channels), filter them in container as directMessages prop
• Limit the number of direct messages displayed on the dashboard (max 10?), but fetch all of them