Add support for favorite logging

[otiala]

1. Create a new TimestampedModel for HobbyFavorite:

• hobby (fk)
• count (int)

2. Add attribute favorite_count (int) for model Hobby.

• Handle increment of favorite_count the way it is handled in Benefit and Promotion

3. Add API endpoints for HobbyFavorite.

• Validate count in serializer to only allow 1 or -1 value.