kivy: support invoices with "max" amount

closes #5781

electrum/util.py

@@ -591,9 +591,11 @@ def chunks(items, size: int):
         yield items[i: i + size]
 
 
-def format_satoshis_plain(x, decimal_point = 8):
+def format_satoshis_plain(x, decimal_point = 8) -> str:
     """Display a satoshi amount scaled.  Always uses a '.' as a decimal
     point and has no thousands separator"""
+    if x == '!':
+        return 'max'
     scale_factor = pow(10, decimal_point)
     return "{:.8f}".format(Decimal(x) / scale_factor).rstrip('0').rstrip('.')