Putting the dropdown content in the details elemenet have been inconsistent.
Transition are not working correctly and different browsers are not handling it well.
Furthermore, the previous implementation does not allow an always visible state of dropdown content.
Fixes: #882
This is a part of the site header refactor
* Use relative units
* Add title to toggle button
* Remove unnessecary CSS
* Rewrite HTML classes
* Temporary remove drawer subsearch, it will be re-explored in a later patch