Date separators push sticky header off screen

[gnprice]

When a date separator is scrolled near the top of the message list's viewport, it pushes the sticky header up off the top of the screen:

In other words it does the same thing that a new recipient header would do. (Once the date separator reaches the top of the viewport, though, the header reappears — this time the one provided by the date separator, so it looks the same as the old recipient header except for its date.)

But a date separator doesn't have all the information a recipient header does, so it doesn't replace the old recipient header and shouldn't push it off the screen. Instead, it should behave the same as a message, and let the preceding item's sticky header overlap into it.

This is a sort of followup to #173 / #469, where we introduced date separators in the first place.