feat(ui): Add custom title functionality (#825)

server/lib/notifications/agents/slack.ts

@@ -58,7 +58,7 @@ class SlackAgent
     payload: NotificationPayload
   ): SlackBlockEmbed {
     const settings = getSettings();
-    let header = 'Overseerr';
+    let header = settings.main.applicationTitle;
     let actionUrl: string | undefined;
 
     const fields: EmbedField[] = [];