Handle narrowing to empty streams in a better way

[preetmishra]

Currently, narrowing to empty streams makes the selection bar disappear. Moreover, it is difficult to make out why nothing is being shown in the MessageBox.

To reproduce:

• Navigate to an empty stream (e.g. Django on chat.zulip.org).
• Press enter.

We could try imitating the web and the mobile app that show a rather informative message, when narrowed, for these streams.

Another aspect, related to narrowing to empty streams, has been discussed in #259.

[neiljp]

@preetmishra This may be resolved by a relatively early PR (#278) from @sumanthvrao? That PR has been sitting around without getting merged for too long, but should be good as a starting approach after rebasing and final reviews. We could potentially take a different approach, of course.

[preetmishra]

@zulipbot add "bug"