Stop services giving the same ID to all their sandboxes

This helps in case of task that require multiple sandboxes (like those
of Communication type) to reduce mutual interferences.
Doc about task versioning.
New testimonial.
Completed zh translation. Thanks to Yuan.
Proposal for detailed timing documentation.
Update documentation about running CMS.
Update documentation about creating contests.
Update documentation about configuring a contest.
Update documentation about task types.
Update documentation about external contest formats.
Update documentation about RWS.
Update documentation about localization.
Update documentation about troubleshooting.
Update documentation about CMS internals.
Fix typos in task versioning documentation.
Fix typos in detailed timing configuration documentation.
Use "they" instead of "she" throughout the documentation.
Update documentation about installation.
Reformulate documentation introduction.