Add whitespace and dividers between messages
Add whitespace and dividers between messages

Webpage is cleaner and easier to read when messages are separated by
dividers and whitespace.

Side effect:
* Remove bottom margin from date range header. Having both the header's
  bottom margin and the first message's top margin creates an unusual
  amount of empty space.

geoffreyyip/mta-dashboard,geoffreyyip/mta-dashboard