presence [nfc]: Use language-level `for` loop instead of .reduce.
presence [nfc]: Use language-level `for` loop instead of .reduce.

It's possible for .reduce to effectively simplify code, but in this
case, we're better off following the web app and doing a good
old-fashioned loop.

Preparation for the next commit, where we fix a bug introduced by
this too-terse/too-clever code.

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