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/mattermost.js

@@ -3,14 +3,14 @@ const axios = require("axios");
 const { DOWN, UP } = require("../../src/util");
 
 class Mattermost extends NotificationProvider {
-
     name = "mattermost";
 
     /**
      * @inheritdoc
      */
     async send(notification, msg, monitorJSON = null, heartbeatJSON = null) {
-        let okMsg = "Sent Successfully.";
+        const okMsg = "Sent Successfully.";
+
         try {
             const mattermostUserName = notification.mattermostusername || "Uptime Kuma";
             // If heartbeatJSON is null, assume non monitoring notification (Certificate warning) or testing.
@@ -98,10 +98,7 @@ class Mattermost extends NotificationProvider {
                     },
                 ],
             };
-            await axios.post(
-                notification.mattermostWebhookUrl,
-                mattermostdata
-            );
+            await axios.post(notification.mattermostWebhookUrl, mattermostdata);
             return okMsg;
         } catch (error) {
             this.throwGeneralAxiosError(error);