message_edit: Change initial default for old thread notifications.
message_edit: Change initial default for old thread notifications.

After some discussion with the community, we've concluded that in most
cases, one probably wants to notify only the destination thread, not
the source location; so let's make that the default.

We preserve the model that we remember what you did during the last
move in a given browser session, which is very helpful if you're doing
an operation N times in a row.

Fixes #21838.

zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip