Keep the redirect working when a URL alias is deleted

Problem/Motivation

This relates to #2821158: Guide users who enter path aliases to use internal paths instead

Redirects are matched against system paths. So a redirect from node/123 also redirects /foo-bar, the alias of that node. Drupal turns /foo-bar into node/123 before it looks for a redirect (inbound path processing).

When the alias is deleted, that stops. The published URL returns a 404 and nothing warns the user.
Deleting the node does the same, because its aliases are deleted with it.

This is usually the point of the redirect: the editor wants /foo-bar (the canonical path) to keep pointing to /somewhere once the node is gone.

Steps to reproduce

1. Create a node (e.g. /node/123) with the URL alias (e.g. /foo-bar).
2. Create a redirect from node/123 to /somewhere.
3. Visit /foo-bar, it redirects to /somewhere.
4. Delete the /foo-bar alias (or the node).
5. Visit /foo-bar, it is a 404.

Proposed resolution

When a path alias is deleted, copy the redirect for its system path to the alias path so the published URL then keeps redirecting.
This is opt-in, behind a new setting (off by default).

Remaining tasks

Review.

User interface changes

A checkbox on the redirect settings form, "Keep redirects working when a URL alias is deleted", off by default.

API changes

None.

Data model changes

None.