Implemented the ability to change the JWT key on runtime. (#217)

* Implemented the ability to change the JWT key on runtime.

* Added .7z file extension support

* Cleanup

* Added Feathub link

* Code cleanup

* Fixed up a build issue on CI

Kavita.Common/Configuration.cs

@@ -0,0 +1,47 @@
+using System;
+using System.IO;
+using System.Text.Json;
+
+namespace Kavita.Common
+{
+    public static class Configuration
+    {
+
+        public static bool CheckIfJwtTokenSet(string filePath)
+        {
+            try {
+                var json = File.ReadAllText(filePath);
+                var jsonObj = JsonSerializer.Deserialize<dynamic>(json);
+                const string key = "TokenKey";
+
+                JsonElement? tokenElement = null;
+                if (jsonObj?.TryGetProperty(key, out tokenElement))
+                {
+                    return tokenElement?.GetString() != "super secret unguessable key";
+                }
+
+                return false;
+                
+            }
+            catch (Exception ex) {
+                Console.WriteLine("Error writing app settings: " + ex.Message);
+            }
+
+            return false;
+        }
+
+        public static bool UpdateJwtToken(string filePath, string token)
+        {
+            try
+            {
+                var json = File.ReadAllText(filePath).Replace("super secret unguessable key", token);
+                File.WriteAllText(filePath, json);
+                return true;
+            }
+            catch (Exception)
+            {
+                return false;
+            }
+        }
+    }
+}