msglist: Friendlier placeholder text when narrow has no messages

[chrisbobbe]

When a message list has no messages, we currently just show "No earlier messages". Screenshots for the narrows currently supported in the message-list page:

Combined feed	Channel	Topic	DM (1:1)	DM (self)	DM (group)	Starred messages	Mentions

This issue is to show a simple message for all message-list pages. Conditional per-narrow logic is tracked in #1653 for later.

For the design, let's aim for consistency with #1552, which is based on the Figma (but excludes some graphics; #1551).

cc @alya

[alya]

"There are no messages here." is the default placeholder on web, which seems more clear ("earlier than what?").

[alya]

But if it's comparably easy to do specific messages depending on the view, I would just copy whatever we have on web. If we don't like some of them, we can discuss that and change the web one too, but it might as well be consistent.

[chrisbobbe]

Makes sense. This may interact with Greg's work on #82 / #80, so I'll hold off on this for now. (Normally I'd go ahead because new-DM #127 will make this more visible.)