message: Simplify getLastTopicForNarrow.
message: Simplify getLastTopicForNarrow.

The old code used a number of Array methods, but they didn't leave the
code super simple; and I think the limited API options contributed to
the temptation to use a NULL_MESSAGE.

I'd love to make this a `for...of` loop, but without an analogue to
Python's `reversed` there doesn't seem to be a way to do that that's
less work than just writing the old-fashioned loop directly.

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