Defaut redirect object should contemplate non-existent routes

[sgobotta]

Describe the bug
As a user, if I don't assign a show view for a Resource, whenever I trigger a submit inside an edit view the router will try to redirect to a non-existent view. This is caused because the redirect object assigned by default assumes a show component will always exist.

To Reproduce
Steps to reproduce the behavior:

1. Instantiate a Resource with every view but a show view
2. Go to your edit view and submit a modification
3. The router will try to redirect to a show view

Expected behavior
Vue-Admin should take into account redirecting to views that doesn't exist.

Suggestions:

• provide internal logic to match the list view when another view doesn't exist?
• redirect to a 404 page?