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/apprise.js

@@ -2,13 +2,14 @@ const NotificationProvider = require("./notification-provider");
 const childProcessAsync = require("promisify-child-process");
 
 class Apprise extends NotificationProvider {
-
     name = "apprise";
 
     /**
      * @inheritdoc
      */
     async send(notification, msg, monitorJSON = null, heartbeatJSON = null) {
+        const okMsg = "Sent Successfully.";
+
         const args = [ "-vv", "-b", msg, notification.appriseURL ];
         if (notification.title) {
             args.push("-t");
@@ -23,7 +24,7 @@ class Apprise extends NotificationProvider {
         if (output) {
 
             if (! output.includes("ERROR")) {
-                return "Sent Successfully";
+                return okMsg;
             }
 
             throw new Error(output);