refs #15191 first scaffold of basic design for bbi

* overwrite basic color variables
* set header logo
* fix css-linter warnings
* initial SCSS fixes (fine tuning)
* adds clarifying name
* properly centers searchbox on home
* branches and cleans up searchbox template
* renames Start into home
* removes language dropdown in favour of display of non-selected language only
* adds svg icons for language selection
* adds initial adaptations for responsive design (header)
* performs general cleanup of SCSS