Option to disable breadcrumb messages were given in both message edit
form and topic edit stream popover.
User now has the option to select which stream to send the notification
of stream edit of a topic via checkboxes in the UI.
This completes the implementation of support for moving a topic to
another stream by adding a basic UI for it.
Fixes#6427, which was previously the most-upvoted issue request in
Zulip.
There are likely to be a bunch of follow-up UI improvements on top of
this change to fully flesh out the feature.