Add a start date to a flag to accompany the expiration date

[ryanirelan]

Flags currently support an expiration date. Add a corresponding start date so a flag can be scheduled to activate at a future point in time.

Motivation

This pairs with the existing expiration date to define a full active window for a flag — the flag would only be considered active between its start date and expiration date.

Proposed behavior

• Add an optional start date field to a flag.
• Before the start date, the flag evaluates as inactive/off.
• Between the start date and expiration date, the flag is active (subject to its other conditions).
• After the expiration date, the flag is inactive (existing behavior).
• Both fields remain optional and independent (a flag can have just a start date, just an expiration date, both, or neither).