R1:deb754dfe173

Make SSH key revocation actually prevent adding the same key back

Summary:
Ref T13043. In an earlier change I updated this langauge from "Deactivate" to "Revoke", but the behavior doesn't quite match.

This table has a unique key on `<isActive, keyBody>`, which enforces the rule that "a key can only be active for one unique user".

However, we set `isActive` to `null` when we revoke a key, and multiple rows are allowed to have the value `<null, "asdf">` (since a `null` column in a unique key basically…