Refactor abort_if() calls to Gate::denyIf() when a user is concerned

[imliam]

Per this thread, the reasons for preferring Gates here are:

• abort_if() throws a plain HttpException, while Gate::denyIf throws a more precise AuthorizationException which can be used by the exception handler
• Gates get their hooks picked up, so admins/etc. won't be tripped up by these guard clauses

- abort_if($user->id === $post->user_id);
- abort_if($post->user()->is($user));
+ Gate::denyIf($user->id === $post->user_id);
+ Gate::denyIf($post->user()->is($user));

This is a quite opinionated one so I wouldn't include it as part of any default sets, but still think it's an interesting and useful refactor.

[GeniJaho]

I feel like this rule could remove some checks that are supposed to prevent all types of users, and there's no way to handle all cases correctly.

For example, abort_if($user->posts()->count() < 100);. This is not a user access permission check, but the rule would change it.