message_edit_form.handlebars already has a message_edit_topic that
refers to the topic edit section of message editing, and this made
things very confusing.
This change would allow anyone in the realm to set a topic for a "no topic"
message. As soon as the message topic is set, only the sender can change it again.
(imported from commit 0a91a93b8fd14549965cedc79f45ffd869d82307)