Allow enterprise urls without a scheme.

Don't insist on `https://enterprise.me`, allow `enterprise.me`. This defaults to `http` rather than `https` but there should be a redirect in place.

Author: grokys

src/GitHub.App/ViewModels/Dialog/LoginToGitHubForEnterpriseViewModel.cs

@@ -141,7 +141,7 @@ async void EnterpriseUrlChanged(string url, bool valid)
 
             try
             {
-                var uri = new Uri(url);
+                var uri = new UriBuilder(url).Uri;
                 ProbeStatus = EnterpriseProbeStatus.Checking;
 
                 if (await enterpriseCapabilities.Probe(uri) == EnterpriseProbeResult.Ok)

src/GitHub.Exports.Reactive/Validation/ReactivePropertyValidator.cs

@@ -280,8 +280,15 @@ public static ReactivePropertyValidator<string> IfNotUri(this ReactivePropertyVa
         {
             return This.IfFalse(s =>
             {
-                Uri uri;
-                return Uri.TryCreate(s, UriKind.Absolute, out uri);
+                try
+                {
+                    new UriBuilder(s);
+                    return true;
+                }
+                catch
+                {
+                    return false;
+                }
             }, errorMessage);
         }