This commit unifies translation strings for reply button label
to reduce translation effort.
Both the `has_empty_string_topic` and other cases now use a single
"Message <z-recipient-label></z-recipient-label>"`, with dynamic
content passed appropriately.
This commit adds support to display `realm_empty_topic_display_name`
value as the compose reply button label for topics having the actual
value of empty string.
Since we allow calling `add_messages` without checking fetch status,
it can lead to non-contiguous message history due to latest message
being added to a message list without previous messages being
fetched.
To fix it, we only allow adding new messages via message_fetch
which properly sets `anchor` to the last message in the list
before fetching and adding messages to the list.
