Gateway: don't generates a new address every time the page is loaded

[justinmoon]

This will cause an excessive number of state machines to be spawned

[wbobeirne]

What's the alternative? We could cache an address in localStorage or something, but then you run into the problem of the implicit expiration of addresses in fedimint. IMO the endpoint should return the same address until it's been used. Maybe add an optional parameter to force a new address if necessary.

I think this is more of a fedimint/fedimint issue than fedimint/ui.