Fix respnsive behavior of titlebar elements

_ Include flexbox mixin in titlebar elements: Header,
  image and playbutton and determine their position according to
  the breakpoints
- Add missing breakpoint to _variable.scss for small screens >360px
Fix position for paragraph in tab footer

- Disable extra padding and margin for correct alignment
  of the paragraph containing the copyright information
- The commented-out padding and margin will be needed again
  when the second paragraph containing the social link
  images will be ready for display.
Delete obsolete styles for tabs in video player
Fix indentation of main headline in tabs