Ability to limit available fiat currencies (#1748)

* feat: limit number of available fiat currencies
* feat: Add allowed currency selector to Admin UI
* motorinas suggestions

---------

Co-authored-by: dni ⚡ <office@dnilabs.com>

lnbits/core/views/generic.py

@@ -18,6 +18,7 @@ from lnbits.helpers import template_renderer, url_for
 from lnbits.settings import get_wallet_class, settings
 
 from ...extension_manager import InstallableExtension, get_valid_extensions
+from ...utils.exchange_rates import currencies
 from ..crud import (
     create_account,
     create_wallet,
@@ -400,6 +401,7 @@ async def index(request: Request, user: User = Depends(check_admin)):
             "user": user.dict(),
             "settings": settings.dict(),
             "balance": balance,
+            "currencies": list(currencies.keys()),
         },
     )