Can't signoff on a rule update that previously didn't require signoffs

[ahal]

We discovered that if you modify a rule that doesn't need a signoff to then require a signoff, the "Signoff" button stays disabled.

STR:
In Balrog stage, create any rule on the Firefox VPN : release-cdntest channel. No signoff is required. Modify the rule and update the channel to release*. Signoff is required for this update as expected.

Expected:
The rule update can be signed off on.

Actual:
The "Signoff" button is greyed out and no one can signoff.

The workaround here is to duplicate the rule rather than modify the old one.