master_seed won't generate if Multiuser plugin is enabled and master_seed isn't exist

Master needs a master_seed in `users.json` to manage a public proxy, if master deletes `users.json` for some reason, ZeroNet will create a new `users.json` which is only contains `{}`.

Start with the Multiuser plugin behavior involving users.json and reproduce the case where users.json is deleted and recreated as {}. Trace how master_seed generation is handled, then verify that a newly recreated users.json allows the master to manage a public proxy.

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