Notices should have an expiry date

[bryank-cs]

It'd be useful if cluster notices had an expiry date so that they would stop appearing as part of the portal (and the API wouldn't report them) after that date/time. This allows for temporary maintenance notices to be taken down after they have passed, for example.

[simonLeary42]

Personally I would prefer if the notices did not expire but did expire from being displayed on the front page