msglist: Use special background color for DM messages

[chrisbobbe]

This is part of #157.

The web app uses a special background color for DM messages:

This color is currently identified as --color-background-private-message-content in web/styles/app_variables.css in zulip/zulip.

We should use this color.

We should follow the web app in also giving this background to a DM message's corresponding date separator, if any:

(Expand to see a screenshot from a quick attempt I made, and _forgot_ to handle the date separator. Don't do this! 🙂)