topic_list: Convert widget to an ES6 class TopicListWidget.
topic_list: Convert widget to an ES6 class TopicListWidget.

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

eeshangarg/zulip,hackerkid/zulip,rht/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,showell/zulip,andersk/zulip,showell/zulip,punchagan/zulip,kou/zulip,andersk/zulip,kou/zulip,kou/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,brainwane/zulip,rht/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,brainwane/zulip,hackerkid/zulip,hackerkid/zulip,showell/zulip,andersk/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,hackerkid/zulip,brainwane/zulip,showell/zulip,rht/zulip,zulip/zulip,rht/zulip,showell/zulip,punchagan/zulip,rht/zulip,showell/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,zulip/zulip,brainwane/zulip,punchagan/zulip,brainwane/zulip,eeshangarg/zulip,brainwane/zulip,kou/zulip,brainwane/zulip,andersk/zulip,zulip/zulip,andersk/zulip,punchagan/zulip