Add support for morph maps

[pelmered]

If you define morph maps for the model that you add comments for it will not work.
This is because you use use getMorphClass() to translate the namespaced model name into the mapped name here, but when you query the database for comments you use static::class

So if you for example have this:

Relation::morphMap([
   'user'           => \App\Models\User::class,
]);

you will insert 'user' into the subject_type column, but then you would use App\Models\User in the query.

With this fix, you will use 'user' in the query as well.

[luke-parallax]

Thanks @pelmered! 🙌