dcrweb - navigation update (2) - breadcrumbs menu

@rmykolas, same as main navigation update, hold off merge till noted.

The breadcrumbs menu will be a second layer navigation within all subpages to simplify navigating inside a category.

Presently selected category name is static.
Hovers use color change to #091440 and for external pages display a 6px external page icon.
Underline indicates the present subpage.
When scaling the listed subpages need to be left aligned after the first subpage.
Spacing 30px above, 50px below as described w/ example content.

decred / dcrdesign