Fix responsive bahavior of titlebar elements

- Delete trouble-causing width and height styles of image
  off medium size screen breakpoints
- Set a wider range of pixels to a breakpoint, that provides
  the full-width image display
- Move playbutton styles from medium size screen breakpoints to
  tiny screen breakpoints to be displayed consistently in all resolutions
Fix margins of play button to fit better in mobile screens
Delete commented line in titlebar.scss
Merge pull request #191 from podlove/fix-mobile-titlebar

Fix responsive behavior of titlebar elements