Add optional expiration date to Syncshells

[universalconquistador]

Some players would like the option to set an expiration date on their Syncshells, after which the Syncshell will be automatically permanently deleted.

• Syncshell expiration date can be optionally enabled and set when creating a Syncshell
• Syncshell expiration date can be seen and adjusted in the Syncshell admin view
• Every minute the server destroys any Syncshells with an expiration date in the past