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
To be consistent with the new naming in the app 'quote_reply' was also renamed to 'quote_message' Fixes part of #31953
Signed-off-by: Anders Kaseorg <anders@zulip.com>