separate private channels and guilds / guild channels

currently, a "direct message channel" or "private channel" is a type of guild that contains a single channel. because of the simplicity of private channels as compared to full-on channels or guilds, I suggest a new `PrivateChannel` type:

```protobuf
// PrivateChannel is a private channel between a list of users, independent of a guild.
// It should be possible to use this channel for both text and voice communications.
message PrivateChannel {
// The list of users that have access to this channel.
repeated uint64 members = 1;
// Whether it is possible to add or remove users to/from this channel.
bool is_locked = 2;
}
```

(note that fully private, or "direct message", channels, have the `is_locked` type set - it is not possible to add more users to a DM between two users.)