replace planet dropdown with fancy overview page (draft)

 * show planet icon, construction times in addition to coords and
 planet names;
 * show planets and corresponding moons side-by-side;
 * applicable both to switching planets and fleet mission destination;
 * add new chooser button directly to page header
 (easier to switch between fleet screens of different planets);
 * leave old dropdowns as is until new chooser will be tested by players.