Use separate face for timestamp-header-with-date vs timestamp-header

I find it useful when scanning the backlog if date changes jump out at me (often I'm checking for discussions from a particular day or range of days); but it's then distracting if every timestamp on account of `ement-room-timestamp-header-delta` has that same contrasting appearance.

I suggest an additional face for the date changes. It can inherit the existing face by default, so it's a no-op for people who don't want these things to be different.

Tangents:

- Perhaps faces should have their own customize group?
- Does the "with-date" header occur as soon as some event has passed the date boundary, or is it still necessary to have an elapsed `ement-room-timestamp-header-delta` number of seconds?