Automatically redirect /amp/ endpoint to ?amp on 404 responses

[westonruter]

Feature description

When using another plugin that tries to use /amp/ for all URLs, switching to this AMP plugin can result in non-singular URLs 404ing. To deal with that, I created a helper migration plugin: https://gist.github.com/westonruter/6b7d3b168711ee03c4ee29b68c3719b2

Nevertheless, this seems like something that would make sense to be part of the the AMP plugin directly. In particular, it would make sense to add after #2204 is figured out.

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Implementation brief

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Demo

Changelog entry

[westonruter]

A related issue came up in #474 (comment).

Namely, if someone originally had /amp/ for pages, upon switching to the AMP plugin these URLs won't work anymore and they may end up getting routed to WordPress's own canonical redirect system, so in that case /some-page/amp/ will redirect to /some-page/ and not /some-page/?amp as expected. While we don't want to register an amp endpoint for pages, we should perhaps hook into WordPress's canonical redirects to check for /amp/ being present, and make sure that WordPress will redirect to ?amp (or ?amp=1 per #2204) instead of removing it from the URL altogether.

[westonruter]

Closing as duplicate of #2062