Remove concept of 'global' channel, context broadcast should always be opt-in

[rikoe]

In FDC3 1.0 there was only a global broadcast function, and in FDC3 1.1 when channels were added, the global channel was added for backwards compatibility reasons.

However, a global channel that apps always broadcast on doesn't really make sense, it doesn't result in a good user experience.

In FDC3 2.0, joining channels (and receiving broadcasts) should always be opt-in. Applications should only be joined to a channel if they are specifically added to add it, either programmatically or via channel selection UX.