This commit adds `allow_empty_name: true` to the following
endpoints to receive empty strings as topic name when such
topics are available:
* `GET /messages`
* `GET /messages/{message_id}`
* `GET /messages/{message_id}/history`
Also, it adds `empty_topic_name` client capability for spectators.
These changes are part of the broader effort to enable
support for empty string as a topic name.
Forward message previously didn't open the recipient dropdown
when forwarding from the combined feed.
This commit will also make the messages keep their original
channel or pm-recipient when forwarding from combined feed, to
be consistent with the behavior in the rest of the app.
Forward message starts a new message with the quoted context
(message or selection).
The compose box opens with then channel picker open and no topic.
Also added documentation about this feature in the help center.
Fixes part 2 of #31953
