[web] whitespace cleanup in http_headers.py

[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.

[devtools] add a 'method' argument to RepoAccess.web_request so that one can easily forge a request with any HTTP method. Also a bunch of clone cleanups.

[web] Fix handling of some http headers The generator takes as argument a list of raw string values, and is supposed to return the parsed version. Calling str on that list makes no sense.

[web/request] deprecate user_callback Storing closures in session data considered harmful. Closes #3567793

[doc] undocument user_callback Related to #3567793

Added tag cubicweb-centos-version-3.17.14-1, cubicweb-version-3.17.14, cubicweb-debian-version-3.17.14-1 for changeset fa00fc251d57

[migractions] Better handle removal of RQLConstraint in sync_schema In rdef synchronisation, filter unmodified constraints before processing the sets of old and new constraints. Add a new `constraint_by_eid` method on RelationDefinitionSchema, which eliminates the ambiguity of `constraint_by_type` usage when several constraints of the same type are involved in the same migration. Closes #3633644.

[test] when the config change, we should cleanup sys.path and sys.modules. Closes #3634164 Without this, modules imported within instance home will be messed up.

[wsgi] make sure request.content is available for consumption The wsgi.input stream is not seekable, so make a copy either to memory or disk so it's still available to the user even after we've parsed it (for POST form processing). Closes #3554996