Fix channel manager to get personal and group channels

[rob05c]

Right now, the channel manager queries 'channels' from the Slack API, which only gets regular channels.

We probably want to get personal and group channels too, and treat them like other channels.
Alternatively, we could make channel managers for each. But things like messages come from the API with channel IDs all together, so it probably makes sense for the client to store and manage them together too.