bot clean: all including the human messages interacting with bot

[mordamax]

https://matrix.to/#/!JUeaZUiYbdrvzvtwSL:parity.io/$EFjxA_eH-NuUWrCFEudUqHXV1-JcmVKQXRHyQ7t478M?via=parity.io&via=web3.foundation

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@rossbulat: It would be pretty cool if bot clean also deleted the commands used to summon the bot. So all the bot clean and bot rebase etc comments would also be removed.
@gpestana: Maybe that behaviour could be requested with bot clean --all
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