conversations: Dedupe a bit of redundant logic in grouping.
conversations: Dedupe a bit of redundant logic in grouping.

Before we switched from timestamps to consistently using message IDs
to sort messages, in 7704c5183, these two branches were different.
Since that commit, though, they're identical; so collapse them into one.

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