DIGGLIKE-167: make Category and DiggSubmitType serializable

It is needed by objects in an ehcache cache because they are evicted to
the disk periodically. If they are not serializable, it generates a lot
of errors

git-svn-id: http://dev.lutece.paris.fr/svn/lutece/portal/trunk/plugins/collaboration/plugin-digglike@84711 bab10101-e421-0410-a517-8ce0973de3ef