Renamed some columns to be more generic and applicable. Removed now redundant book_id column. Allowed nullable entity morph columns for non-entity activity. Ran tests and made required changes.  | 
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| .. | ||
| Activity.php | ||
| ActivityService.php | ||
| ActivityType.php | ||
| Comment.php | ||
| CommentRepo.php | ||
| Tag.php | ||
| TagRepo.php | ||
| View.php | ||
| ViewService.php | ||