Issue title: Doesnt work with custom user model primary key
 Issue body: I have a `user_id` primary key for the user model and on clicking the impersonate button, I get redirected to `/nova-impersonate/users/null?redirect_to=/` that results in a 404.

I see that the code uses `$user->id` in many places and should be using `$user->getKey()` instead for this to properly work.
 Comments: 
 Comment 0: can u submit a PR fix?
 Comment 1: Done. Cheers and thanks for the excellent package! :)
 Comment 2: @KABBOUCHI would appreciate if you could tag a new release so that I can use it in my app. Thanks! 👍 
{'source': 'GithubIssues', 'repo_name_with_owner': 'KABBOUCHI/nova-impersonate', 'issue_id': 388919898, 'repo_stars': 227.0}