Redesign party commands

It may be better to send party commands using a single command with many subcommands than our current implementation of having a separate command for every action.

We could go with a "polymorphic" approach:
```
{
"command": "party",
"action": "invite",
"recipient_id":
}
```

Or more of a "compositional" approach:
```
{
"command": "party",
"action": "invite",
"data": {
"recipient_id":
}
}
```

The compositional approach has a little more boilerplate built into the message, but it feels cleaner to me.