changelog: Add issue link for larger avatars.
changelog: Add issue link for larger avatars.

Missed this on the first pass because no issue got closed by this
change; we had #3515, but we'd closed it before deciding to make
the change after all.

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