[components] refactor main template header: make it much more flexible by using CtxComponent * rename userlink class (not private) with somewhat bw compat * make some cosmetic adjustements on log-in/registration components * update to component api, though this may breaks some sub-classes (for application customizing the breadcrumbs component for instance)

doc/book usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc