[web] implement cross origin resource sharing (CORS) (closes #2491768)
Partial implementation that is enough to get started but leaves out
some of the advanced features like caching and non-simple methods
and headers.
Startup views
-------------
Startup views are views requiring no context, from which you usually start
browsing (for instance the index page). The usual selectors are
:class:`~cubicweb.predicates.none_rset` or :class:`~logilab.common.registry.yes`.
You'll find here a description of startup views provided by the framework.
.. automodule:: cubicweb.web.views.startup
Other startup views:
*schema*
A view dedicated to the display of the schema of the instance
.. XXX to be continued