Fix DetachedInstanceError after StatusCodeRedirect

In certain cases, e.g. after an exception has been thrown, the database
session may be detached, but database objects managed by the orm are
still refered to from the same thread.

Such objects can be merged with the new db session, as described in
http://stackoverflow.com/a/9705458/201743

This is done with `c.user` which is re-retrieved from repoze.who after a
StatusCodeRedirect.

This fixes what ade22d799ebe894b1f7a9fbc3a5685e6aceca4e7 works around.

See also discussion on: https://bitbucket.org/phihag/hhu-
adhocracy/issue/168/error-handler-causes-is-not-bound-to-a