IDEA: Hide stream name in message recipient header, in stream narrow?

[neiljp]

This is split out from #719.

This concerns removing redundant information, much like #1343, so could also simplify the UI.

However, this could lead to a less consistent user experience, in that the recipient headers would look different in other interleaved narrows, including all-messages, mentions and starred messages narrows, where the stream name would be necessary.