FIX: perform `agree_and_keep` action only if possible. (PR #13967)

While deleting spammers from flag modal it’s trying to perform agree_and_keep action where it’s not possible (or already performed).


We need a test here :slight_smile:

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Could this still fail if @actor doesn’t have enough permissions, even if the system user can agree with the flag? Maybe we should check using @guardian instead?

it’s done.

Looks great. Thanks!