dropdown_list_widget: Convert DropdownListWidget to ES6 class.
dropdown_list_widget: Convert DropdownListWidget to ES6 class.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

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