add time zone support for sites and users

Oqtane.Client/Services/TimeZoneService.cs

@@ -0,0 +1,22 @@
+using System.Collections.Generic;
+using System.Net.Http;
+using System.Threading.Tasks;
+using Oqtane.Documentation;
+using Oqtane.Models;
+using Oqtane.Shared;
+
+namespace Oqtane.Services
+{
+    [PrivateApi("Don't show in the documentation, as everything should use the Interface")]
+    public class TimeZoneService : ServiceBase, ITimeZoneService
+    {
+        public TimeZoneService(HttpClient http, SiteState siteState) : base(http, siteState) { }
+
+        private string Apiurl => CreateApiUrl("TimeZone");
+
+        public async Task<List<TimeZone>> GetTimeZonesAsync()
+        {
+            return await GetJsonAsync<List<TimeZone>>($"{Apiurl}");
+        }
+    }
+}