Remove Django-Taggit completely.

We aim to replace the use of django-taggit with a custom implementation for managing tags in the Shift model. Below are the required steps:

### **Refactor the Shift Model**
1.Remove the django-taggit attribute from the Shift model.
2.Introduce a new foreign key relationship to a newly created Tag model.
3.Ensure proper handling of duplicate tags to maintain data integrity.

### **Data Migration**
1. Create a migration script to transfer existing tag data from django-taggit to the new schema.
2. Verify that all historical data is accurately migrated and adheres to the new structure.

### **Update Codebase**
1. Update the Shifts model usage in the ShiftSerializer and ShiftViewSet.
2. Modify relevant views, such as ContractViewSet, UpdateReports, and any others that interact with the Shift model tags.
3. Revise and adapt affected test cases to reflect the updated implementation.
4. By implementing these changes, we aim to streamline tag management and improve maintainability across the codebase.