Require version numbers for approved intents

[jpmedley]

If a feature has an Intent, then it should have an appropriate release version. An intent to experiment should have values in OT start and OT end fields. An intent to ship should have values in shipping milestone fields.

• Ability to send an intent should be blocked pending provision of a milestone number. This allows developer relations to begin planning as soon as there's a possibility of something shipping.
• To support the desire to under promise and over deliver, feature owners should have the option of hiding the milestone from everyone except Chrome owners and Developer Relations.
• Feature owners should be allowed to enter target version numbers
• Feature owners should be discouraged from entering late numbers. For example, a feature owner who reasonably thinks they can finish something in Chrome 97 should be discouraged from entering Chrome 98 "just in case".
• Version numbers need to go away or not be shown if a milestone is reached and a feature's intent is not approved.

[jpmedley]

@jstenback Here's the ticket we talked about yesterday with a set of proposed requirements.

[jstenback]

This generally looks like a good starting point, even if the details may evolve as we implement this and roll it out and learn more about what would work best here from various stakeholders' perspectives.