XEP-0280: Don't send v1 carbon copies back to the sender

This PR applies three somewhat related changes to `mod_carboncopy`:

1. An earlier version of XEP-0280 specified the `<received/>` and `<sent/>` tags to be siblings of the `<forwarded/>` element, whereas the current version mandates them to be parents of `<forwarded/>`.  `mod_carboncopy` supports both variants.  However, the check that makes sure clients won't receive a copy of the messages they sent didn't work for the old-style schema.  This is now fixed.

2. The `mod_carboncopy:is_carbon_copy/1` function now returns true not only for `<sent/>`, but also for `<received/>` carbon copies.

3. The `mod_carboncopy:check_and_forward/4` function is simplified: It now uses `is_carbon_copy/1`, and multiple `case` clauses arecombined into a single one.