Ability to limit available fiat currencies (#1748)

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

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

lnbits/core/views/api.py

@@ -672,6 +672,12 @@ async def api_perform_lnurlauth(
 
 @core_app.get("/api/v1/currencies")
 async def api_list_currencies_available():
+    if len(settings.lnbits_allowed_currencies) > 0:
+        return [
+            item
+            for item in currencies.keys()
+            if item.upper() in settings.lnbits_allowed_currencies
+        ]
     return list(currencies.keys())