Add user interface for doing site validation of the various templates from the AMP Settings screen

[westonruter]

Breaking out from #1007 which focused specifically on using WP-CLI to do validation of an entire site.

Just as a user can run wp amp validate-site via WP-CLI to do bulk validation on the URLs of a site to check compatibility, there should also be a way to initiate this same validation via the admin.

As noted in #1007 (comment):

Once we have the WP-CLI command in place, then we can look at how to present that same information in the admin. We'll also then need to figure out a good way to use admin-ajax to reliably make requests to batch validate URLs without timing-out. But these things would essentially be GUI improvements over the raw work performed by WP-CLI, so good to focus on that first. On that note, most of the WP-CLI commands should be invoking logic in AMP_Validation_Utils which can be re-shared with the admin integration.

See also #1007 (comment) and other comments on that issue.

Low Priority: Also related to #1273 as it would be helpful to automatically report which templates are best suited for serving as AMP based on validation results.

[westonruter]

This is going to be achieved as part of the site scan feature: #4795 and #4719