Add "View AMP" link to admin bar on non-AMP page in Reader mode

[westonruter]

A very common support request is confusion about why adding /amp/ to the URL leads to a 404 instead. This is normally due to trying to access the AMP version of a Page or some other hierarchical post type, for which ?amp is used for the AMP URL. (See also #2062 to automatically redirect from /amp/ to ?amp when is_404().) When users do not have the AMP Validator extension installed, they will not see the AMP link in the browser extension button. If users are unfamiliar with the amphtml link in the head, they will have no idea how to find the AMP version or even if there is an AMP version available.

So we should consider adding the AMP link to the admin bar for AMP-enabled posts, even in reader mode:

Aside: We should consider eliminating the /amp/ in favor of ?amp everywhere. The endpoint slug is unnecessary for SEO, and it improve consistency when switching between the paired modes (transitional and reader). Additionally, there will no longer be a need to flush rewrite rules. See #2204.