Add start | end date for FEATURE flags

[compojoom]

Sometimes we need to enable a feature flag at a particular time. Other times we need to disable a feature flag at a particular time. I think that it would be beneficial if feature flags had a date field for start and end values. By default they should be null, which means the feature is null. If a date is entered then we should check the current date against the start and end time to determine if the feature is on or off.

Right now someone with access to the config service needs to be available to press a button at a specific time...

[parius]

Equally, someone needs to be present to adjust the date for feature flag.

Imagine the following: Feature is scheduled to go live 14:00. At 13:55 there is an event preventing feature to go live (critical bug found by QA, marketing is not ready, message from legal). If no one with access is available, the feature goes live.