Auto Expiring Flags

[dazoakley]

See http://matt.chadburn.co.uk/notes/apoptosis.html

Add an expiry date to features (by default) so that they automatically turn themselves off after a given time - this then provokes the following questions:

• Did anyone notice it's gone?
• Do people use the feature?
• Should it still be here?

If the answer to any of the above is "no", then the feature should be considered for removal.