Refactor CSS for pane layout

 - remove JS hack to have fixed elements on a certain size,
   elements now use position absolute/relative and floating

 - header, breadcrumbs and panes do not overlap anymore, which
   lead to alot of bugs like visible but not clickable buttons
   and problems when trying to add new elements to the header

 - header is now aligned with navigantion/burger-height
   (both 64px)

 - componentize footer for all_dbs page

Parts of this branch are based on a prototype from Sean Barclay
Sean Barclay <fudd1011@hotmail.com> (@seanbarclay)

Closes: COUCHDB-2320 COUCHDB-2271 COUCHDB-2471 COUCHDB-2482