Private Email Service Support (#1028)

* Added ServerSettingKey's for SMTP and moved email service code to Kavita. Nothing integrated in the UI yet.

* Undo all the custom SMTP stuff and prepare for custom email service url.

* Foundation for email service to use a custom url is setup.

* Implemented the ability to hook up custom email url

UI/Web/src/app/admin/_models/server-settings.ts

@@ -8,4 +8,5 @@ export interface ServerSettings {
     enableOpds: boolean;
     baseUrl: string;
     bookmarksDirectory: string;
+    emailServiceUrl: string;
 }