Adds proper url for redirecting to on page deletion.

When user deletes a page the cms redirects to a hardcoded url.
This PR introduces logic that if the page has a parent it redirects to that parent, otherwise it redirects to root.

@FinalAngel the url is properly set in the ```data-on-close``` attribute, but the javascript keeps redirecting to the hardcoded `/` please look into this.

I'll add tests once frontend work is finished.

Fixes #2118