Changes

The default implementation of the Contribution#canBeAccessedBy(User)
uses the AuthorizationRequestCache to avoid a redondent computation of
the authorization of the user to access the contribution along the same
thread. The cache key was computed from the local identifier of the
contribution which can lead hidden bugs like for example the access to
another contribution of different type with the same identifier value
not being computed. Now the cache key is computed from the global
identifier of the contribution.