Implement withdraw "backdoor" for accidentally sent tokens to community vaults

[0x-r4bbit]

Once #31 is implemented, the vaults will essentially have their own accounting.
This however, obviously doesn't prevent anyone from sending tokens to the vault contract directly.

Assuming that the vault's transferERC20 and transfer721 tokens will operate on the internal accounting, tokens sent directly to the contract without the dedicated deposit functions will lock those tokens forever.

This is why we should consider adding some sort of "backdoor" that enables community owners (or who ever has the privelege), to withdraw any excess tokens from the vault.

Feel free to comment here on why this might not be a good idea.

@0xb337r007 @3esmit