msglist: Use distinct background colors for DM messages.#1305

E-m-i-n-e-n-c-e wants to merge 1 commit intozulip:mainzulip/zulip-flutter:mainfrom

E-m-i-n-e-n-c-e:DmBackgroundE-m-i-n-e-n-c-e/zulip-flutter:DmBackgroundCopy head branch name to clipboard