Improve navigation sub-menu and tabs effects (#2971)

Improve navigation sub-menu and tabs effects (#2971)

* Improve nav border and colors on active and non-active

* Remove margin-top effect on active nav

* Use opacity / bold instead of color change on nav

* Remove dropdown items label in sub-menu and add a class active

* Position sub-menu to fixed

* Autoclose dropdown sub-menu only on outside click

* Remove open dropdown on hover in sub-menu

* Show reusable h1 for dropdown item in sub-menu

* Put reusable sub-menu h1 styles to mixins

* Add icons to sub-menu dropdown-item h1

* Make all the sub-menu accessible with focus

Co-authored-by: kimsible <kimsible@users.noreply.github.com>