Make sure URL base doesn't start with http or https

Closes #532

src/NzbDrone.Api/Config/HostConfigModule.cs

@@ -42,6 +42,9 @@ namespace NzbDrone.Api.Config
                            .ValidIp4Address()
                            .NotListenAllIp4Address()
                            .When(c => c.BindAddress != "*");
+
+            SharedValidator.RuleFor(c => c.UrlBase)
+                           .ValidUrlBase();
         }
 
         private HostConfigResource GetHostConfig()

src/NzbDrone.Core/Validation/RuleBuilderExtensions.cs

@@ -62,5 +62,10 @@ namespace NzbDrone.Core.Validation
         {
             return ruleBuilder.WithState(v => NzbDroneValidationState.Warning);
         }
+
+        public static IRuleBuilderOptions<T, string> ValidUrlBase<T>(this IRuleBuilder<T, string> ruleBuilder)
+        {
+            return ruleBuilder.SetValidator(new RegularExpressionValidator(@"^(?!\/?https?://[-a-z0-9.]+)", RegexOptions.IgnoreCase)).WithMessage("Must be a valid URL path (ie: '/sonarr')");
+        }
     }
 }