changes footer color on mobile to steel
adapts the way less is used to the revised project structure

* merges _less-mixins.less and _helpers.less
* loads _helpers.less right after _normalize.less to make variables and
  mixins available to all other less files -> makes import in individual
  files redundant
* updates less-help.md
mobile menu: implement in css3

- js mobile menu integration removed
- html markup changes (toggle checkbox + 2 labels)
- css styling in new file: _mobilemenu.less
no need for full-javascript mobile menu

- mv p3.mobilemenu.js --> old.mobilemenu.js
mobile menu: js to move search form

- move .search-form into #main-nav on mobile (to be seen)
  and back into .tools on desktop
- file p3.mobilesearchform.js
- uses matchMedia
- integration in action-template-main.js
update compiled files
removes deprecated mixins file
cleanup _header.less from not used mobile menu styles
update compiled files