More options to Custom Amount Pay button (#948)

* Start adding more options to Custom Amount Pay button

This allows you to simplify the custom amount pay button to remove the big + & - buttons along with set a min, max and step amounts. There's also an option to fit the button next to the input amount to have it more condensed(not finished)

* make fit button inline work nicely

* make currency dropdown more obvious

* fix space

BTCPayServer/Models/StoreViewModels/PayButtonViewModel.cs

@@ -22,6 +22,10 @@ namespace BTCPayServer.Models.StoreViewModels
         public decimal Max { get; set; }
         public decimal Step { get; set; }
 
+        // Custom Amount properties (ButtonType = 1)
+        public bool SimpleInput { get; set; }
+        public bool FitButtonInline { get; set; }
+
         [Url]
         public string ServerIpn { get; set; }
         [Url]