[#879] Fix wrapping of long journal titles in Drifting

* set a min-height instead of absolute height for the header, which lets
  the header expand around text instead of abrutply running out

* make sure that header encloses .header-left instead of having its own
  height (to avoid instances of the header left image becoming
  disconnected from the header)

* set the background color of the header to match the topmost row of the
  header image, for a seamless transition upward once we run out of
  image, and then make sure we only repeat the header background image
  horizontally

* only show the right header on big screens, to avoid having it encroach
  on the title

Fixes #879.
Merge pull request #896 from afuna/drifting-journal-title-bug-879

[#879] Fix wrapping of long journal titles in Drifting