Allow slugs or placeholders for page paths

Problem/Motivation

In order to override the node view page, you must use /node/% (or node/%).
If you try to use the more semantically correct /node/{node}, it will not work, even if that's what is used internally.

Proposed resolution

Allow either form.

Remaining tasks

Ensure this doesn't complicate #2549997: Check for replacing existing routes should not remove default arguments
Ensure this works for non-overridden pages as well.

User interface changes

N/A

API changes

API addition

Data model changes

N/A