[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)
[schema / sources] store data sources as cubicweb entities
this implies several changes:
* new CWSource / CWSourceHostConfig entity types
* only the system sources and default admin login/password stored in sources file
(other stuff will be ignored)
* on startup, get sources definition from the database
* every entities have a cw_source relation
* a facet allow filtering