TopicItem: Show the topic in a toast when long-pressed.
TopicItem: Show the topic in a toast when long-pressed.

This is part of #3631, providing a way to see the full topic name
when it's long and gets truncated in the UI.  This commit applies
to topics seen in the unreads screen.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile