Chore: General notification reformatting (#3182)

- I unified where in file the name of `NotificationProvider.name` is placed
- I made sure that all the providers adhere to the signature of `NotificationProvider.send()`
- I made sure that all the providers use `okMsg` if returning success messages directly from the function.
  Here a discussion should be had:
  Should this be refactored into a constant of `NotificationProvider`? I could imagine that `NotificationProvider.SENDING_SUCCESSFULL`  could be a suitable alternative.
- I made sure all providers have the URL they `POST`/`GET` to be extraced into a variable.
  => refactored this way due to Nelsons suggestion

server/notification-providers/smtp.js

@@ -4,13 +4,13 @@ const { DOWN } = require("../../src/util");
 const { Liquid } = require("liquidjs");
 
 class SMTP extends NotificationProvider {
-
     name = "smtp";
 
     /**
      * @inheritdoc
      */
     async send(notification, msg, monitorJSON = null, heartbeatJSON = null) {
+        const okMsg = "Sent Successfully.";
 
         const config = {
             host: notification.smtpHost,
@@ -76,7 +76,7 @@ class SMTP extends NotificationProvider {
             text: body,
         });
 
-        return "Sent Successfully.";
+        return okMsg;
     }
 
     /**