In the spirit of DRY: centralized IsAdmin() check in an extension method

Made sure that non-admins can't trigger the enable/disable URL tracker endpoint
Renamed "admin" in GetEnableState to "isUserAdmin" for clarity

src/Umbraco.Web.UI.Client/src/views/dashboard/developer/redirecturls.controller.js

@@ -15,7 +15,7 @@
             searchTerm: "",
             loading: false,
             urlTrackerDisabled: false,
-            admin: false
+            userIsAdmin: false
         };
 
         vm.pagination = {
@@ -43,7 +43,7 @@
             vm.dashboard.loading = true;
             return redirectUrlsResource.getEnableState().then(function (response) {
                 vm.dashboard.urlTrackerDisabled = response.enabled !== true;
-                vm.dashboard.admin = response.admin;
+                vm.dashboard.userIsAdmin = response.userIsAdmin;
                 vm.dashboard.loading = false;
             });
         }

src/Umbraco.Web.UI.Client/src/views/dashboard/developer/redirecturls.html

@@ -6,7 +6,7 @@
 
         <umb-editor-sub-header-content-right>
 
-            <umb-editor-sub-header-section ng-if="vm.dashboard.admin === true">
+            <umb-editor-sub-header-section ng-if="vm.dashboard.userIsAdmin === true">
 
                 <button
                     ng-if="vm.dashboard.urlTrackerDisabled === false"