mobile menu position + close button + formatting

- moved code from _header.less to _mobilemenu.less
- indentation to 4 spaces
- mobile menu label moved up in the dom
- breakpoints at 320px and 410px
  < 320px: menu spans the whole width, close buttons is shown
  320px >= x < 410px: menu icon is shown on left, close button not
                      shown, max menu with: 250px
  >= 410px x < 768px: same but max menu with: 350px
  >= 768px: desktop view
mobile menu transition effect
mobile menu fix margin of submenus
mobile menu rename nav img files
donate button in the navigatin bar gets margin on top.
recover nav-arrow
shows recent supporters on mobile