base: header enhancements

* Adds endpoint values to url_for for header links.

* Submit link now points to literature submission.

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>
base: signup_base template

* Adds `signup_base` template.

Signed-off-by: Georgios Kokosioulis <giokokos@gmail.com>