refs #15988 [fid_adlr] header

* simplify header
** rename header-text to home-text and move to home.phtml
** no fixed header on home page, except for mobile
** use a non-changing search form on home page
** set background image for main on home page
** add more padding for navigation bar
** refine links on mobile navigation overlay
** refactor searchTabs W3C valid: remove errors
** fix bug of search filters in mobile view: close after click
* style header background and menues for medium to large screens
** remove duplicate classes like additional-text-lg
** re-adjust allocation for md, lg and xl of header and menues
** never hide find button
* implement hybrid 'tablet' view
** also set alert colors
** add some basic transitions
* fix links / placeholders beneath home text and handle link styling (hover / active)
* add background image gimmicks semicircles
** set on home page
** set on non-home page
* remove duplicate css / unused variables