Datetime picker and small edit UI changes (#647)

* do not allow negative amounts for crowdfund and payment requests

* remove currency placeholder in payment requests

* Improve date picker ui 

Clear button only appears when a value is set. If no value is set, display a placeholder indicating it. closes #625

BTCPayServer/Models/AppViewModels/UpdateCrowdfundViewModel.cs

@@ -46,6 +46,7 @@ namespace BTCPayServer.Models.AppViewModels
         public string TargetCurrency { get; set; } = "BTC";
 
         [Display(Name = "Set a Target amount ")]
+        [Range(0, double.PositiveInfinity)]
         public decimal? TargetAmount { get; set; }