Feature Request: Public and Private currencies

[deefactorial]

Stewards get emailed when someone joins a currency they are a steward of.
Stewards get emailed when someone creates a currency in a space they are a steward of.
Stewards get emailed when someone creates a space in a subspace they are a steward of.

The steward is given the option to deny or allow the creation of that account in that space.

Private currencies, and spaces need permission from the steward first before they can transact.
Public currencies and spaces do not need permission from the steward to transact but they may be revoked at any time.

[deefactorial]

stewards now get emailed when someone creates a tradingname, currency or space with the options to enable or disable.

what's left is public and private trading names, currencies, and spaces.