Add Gateway "Pay Self" Lightning support

[Kodylow]

Fedimint just merged having the gateway pay itself by just creating a normal invoice, this needs to get added to the UI for the gateway under the "Lightning" Wallet Action:

Dev steps:

1. Fork and clone UI repo
2. Run nix develop
3. Run just gateway to start up a dev federation and the gateway with a UI
4. Add the "Lightning" wallet action back
5. User should be able to input an amount in Sats
6. User clicks "Create Invoice"
7. User presented with the Lightning Invoice as a QR code with a copyable string below it

[Kodylow]

@0xZaddyy