Adjust Messages top area when in topic & direct message narrows

[neiljp]

When in a topic narrow or a direct message narrow (with specific users, not all direct messages), it would be beneficial to hide the 'Current message recipients' line and content in the line below it.

While in such narrows:

• the content in that line never changes while within the narrow
• the content currently shown in the line above isn't clear/necessary/distinct
  • eg. 'zulip-terminal: topic narrow' with 'zulip-terminal > some topic' beneath it
  • eg. 'Group private conversation' with 'You and [list of user names]' beneath it

To resolve this, when in such a narrow, it would be clearer to

• not show the 'Current message recipients' and the line below
• replace the text we currently have in the top (before the search) by the text that is currently in that line below

In other narrows we should retain the current behavior.

Implementing this could be simple or challenging. A quick exploration may be simple, but adding/updating tests may be difficult due to the way the UI widgets are currently laid out.

Sketching this out, this could look like:

Direct message narrow:

----------------- Messages ---------------
You and user 1, user 2  Search [/]:
------------------------------------------

Topic narrow:

----------------- Messages ---------------
zulip-terminal > some topic  Search [/]:
------------------------------------------

Stream narrow:

----------------- Messages ---------------
zulip-terminal  Search [/]:
------- Current message recipients -------
zulip-terminal > some current topic
------------------------------------------

All Direct messages narrow:

----------------- Messages ---------------
All direct messages  Search [/]:
------- Current message recipients -------
You and user1
------------------------------------------

[srdeotarse]

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[zulipbot]

@srdeotarse You have been unassigned from this issue because you have not made any updates for over 14 days. Please feel free to reclaim the issue if you decide to pick up again. Thanks!

[rsashank]

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[zulipbot]

@rsashank You have been unassigned from this issue because you have not made any updates for over 14 days. Please feel free to reclaim the issue if you decide to pick up again. Thanks!

[Niloth-p]

Related: #1016