[web/request] deprecate user_callback

1  * The implementation of cascading deletion when deleting `composite`
2    entities has changed. There comes a semantic change: merely deleting
3    a composite relation does not entail any more the deletion of the
4    component side of the relation.
5 
6 +* ``_cw.user_callback`` and ``_cw.user_rql_callback`` are deprecated.  Users
7 +  are encouraged to write an actual controller (e.g. using ``ajaxfunc``)
8 +  instead of storing a closure in the session data.
9 +
10 
11  Deprecated Code Drops
12  ----------------------
13 
14  * session.hijack_user mechanism has been dropped.

15          """
16          def rqlexec(req, rql, args=None, key=None):
17              req.execute(rql, args, key)
18          return self.user_callback(rqlexec, rqlargs, *args, **kwargs)
19 
20 +    @deprecated('[3.19] use a traditional ajaxfunc / controller')
21      def user_callback(self, cb, cbargs, *args, **kwargs):
22          """register the given user callback and return a URL which can
23          be inserted in an HTML view. When the URL is accessed, the
24          callback function will be called (as 'cb(req, \*cbargs)', and a
25          message will be displayed in the web interface. The third