Allow editing of first_notification_milestone for current release.

[jrobbins]

The Enterprise team requested the ability to edit the first_notification_milestone even if that affects the release notes for the current milestone. Previously, the server-side implemented a rule that prevented using any first_notification_milestone that used the milestone that was currently on the stable channel.

In this PR:

• Change is_update_first_notification_milestone() to allow entering milestone M or editing an existing value M, unless M+1 has a stable_date in the past. This means that M's stable date can be current or future.

Note that the client-side warnings are unchanged. They never prevented users from entering any value, and it is probably still a good idea to keep the warning to discourage adding and removing items from the currently-live release notes.