Add messaging capabilities

[barelyknown]

Need to add policies for channel, message, and channel subscription.

• Channel
• Messages
• ChannelSubscription

Resources

• Channel
• Messages (from a player (maybe polymorphic) to other players (or more broad?))
• ChannelSubscription

Channel

• status (enum) *required
• name
• description
• messages

Message

• channel *required
• author (polymorphic) *required
• body *required
• created-at

ChannelSubscription

• channel
• subscriber

Features

• Friends
• Emoji
• Markdown formatting
• Notifications
• Muting
• Presence