base: refine of header.html

* Uses semantic markup to provide better readability for crawlers.

* Highlights active elements based on the current blueprint.

* Removes unused code and switched "Sign in" button to redirect
  to the login form without displaying any modal.

* Adds dropdown on the user button with the option to logout.

* Makes all links translatable.

Signed-off-by: Georgios Kokosioulis <georgios.kokosioulis@cern.ch>