Add causer- and subject-type scopes to Activity model

[npostman]

I added 2 scopes to the Activity model for convenience. You can use these scopes to fetch only the activity for subjects of a certain class, or caused by a certain class.

$user_activity = Activity::causedByType(User::class)->get();
$admin_activity = Activity::causedByType(Admin::class)->get();

or

$activity_on_articles = Activity::forSubjectType(Article::class)->get();
$activity_on_posts = Activity::forSubjectType(Post::class)->get();