feat: add discord and pushover as notification agents (#300)

fix: most error messages of the notifications endpoints would not reach the frontend

migrations/000017.php

@@ -0,0 +1,19 @@
+<?php
+
+/* 
+* This migration adds tables to store the date about the new notification methods (pushover and discord)
+*/
+
+/** @noinspection PhpUndefinedVariableInspection */
+$db->exec('CREATE TABLE IF NOT EXISTS pushover_notifications (
+    enabled BOOLEAN DEFAULT 0,
+    user_key TEXT DEFAULT "",
+    token TEXT DEFAULT ""
+)');
+
+$db->exec('CREATE TABLE IF NOT EXISTS discord_notifications (
+    enabled BOOLEAN DEFAULT 0,
+    webhook_url TEXT DEFAULT "",
+    bot_username TEXT DEFAULT "",
+    bot_avatar_url TEXT DEFAULT ""  
+)');