Add support for Action Channels that support targeting messages in multiple channels.

[rrbrambley]

Since message ids are unique across channels, there isn't technically a reason why a single action channel cannot be used for multiple target channels.

We could either:

1. Not require a channel_id in the com.alwaysallthetime.action.metadata channel annotation, or
2. Have an alternate key – channel_ids that is an array of channel ids to be used with the action channel.

[rrbrambley]

Still on the fence about this. Removed the notion of 'target channel' in this branch: https://github.com/rrbrambley/MessageBeast-Android/tree/enhancements/no-target-channel

I think there might be some value in keeping the 'channel_id' key in the metadata annotation because if you look at an action channel independently, it allows you to see which Channel the messages correspond to.