Fixes #36265 - Make redirect when editing host tad more reliable

When the form is submitted, we do a POST via ajax. The post the
redirects to host details, JS follows the redirect and the success
callback is executed only after the redirect is followed. This turned
out to be a dead end.

I was hoping we could access the Location header from the redirect, but
as we get control after the redirect is followed, this doesn't seem to
be possible.

What we however can do, is make the backend return back the path as a
custom header. I cannot say I'm happy about it, but it is probably the
most reliable option we have right now and it works for both the new and
old host details page.

Follows changes introduced in f13142e9e