feat: added follow and unfollow notification (#74)

#### Related Issue (Link to issue ticket)
---

#73 

#### Description
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#### Context
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#### How Has This Been Tested? Screenshots (if appropriate - Postman,
etc)
---

- pytest

#### Types of changes
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the boxes that apply: -->

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
      ​

#### Checklist:
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- [x] I have added tests to cover my changes.
- [x] All new and existing tests passed.

api/v1/services/user.py

@@ -362,6 +362,11 @@ def follow_user(self, db: Session, user_id: str, user: User):
         if followee not in user.followings:
             user.followings.append(followee)
 
+            notification = Notification(
+                user_id=followee.id, message=f"{user.username} followed you"
+            )
+
+            db.add(notification)
             db.commit()
 
     def unfollow_user(self, db: Session, user_id: str, user: User):
@@ -376,6 +381,12 @@ def unfollow_user(self, db: Session, user_id: str, user: User):
             )
 
         user.followings.remove(user_to_unfollow)
+
+        notification = Notification(
+            user_id=user_to_unfollow.id, message=f"{user.username} unfollowed you"
+        )
+
+        db.add(notification)
         db.commit()
 
     def followers(self, db: Session, user: User):