[REFACTOR] Deprecate JIDConverter, use UserConverter instead

All activities no longer need to convert User objects to JID object and
vice versa. This patch introduces a UserConverter that XStream can use
to directly serialize and deserialize User objects.

Change-Id: I10f99ddcf24b64b79466f04f4a572f42e8fe5a2c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1523
Tested-by: Jenkins CI
Reviewed-by: Arndt Lasarzik <alasarzik@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>