feedback feature for Saral Ai

[suryagit-31]

1. We can accept Feedback from users for every paper.
2. Feedback from a user is accepted only once per paper. (To avoid Spam from Same user )
3. Add the MONGODB_URI string to the .env file to enable feedback storage.
4. Example: MONGODB_URI=mongodb+srv://yourname:connection@saralai.p7xxx.mongodb.net/dbName
5. The default collection name is saral_feedback. , you can alter it if needed .