stream_settings: Extract function to set title in right column.
stream_settings: Extract function to set title in right column.

This is a prep commit such that we can use this function to
live-update the title.

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