Add a 'time' limit

We currently have limits on content, but a limit on time that an XWiki instance is to be available would also be interesting.

Various options would exist on enfocring this limit, once the deadline arrives:

• setting the wiki into read-only mode
• redirecting the view action to a custom 'time limit expired' page
• etc.

However, a first thing that could be done would be to just be able to set such a limit and read it through the limits app's script service.