Add ability to look up spec by name

[westonruter]

Feature description

We need to more efficiently walk over the generated spec data, in particular to be able to find tag spec by a spec name. This will allow fleshing out validation errors in the UI with the full context and provide the user with information on amp.dev for more details.

Relates to #1420 and #4566.

• Separate extension specs from tag specs. This eliminates having to look up extension specs from among tag specs.
• Add ability to get a specific spec by name for a given tag, and use in the style sanitizer.
• Add assertion that link rel=stylesheet for fonts spec is present.
• Include extension_type in generated spec data in order to determine if an extension is a custom-template rather than custom-element.
• Remove obsolete/unused AMP_Rule_Spec::$additional_allowed_tags.
• Eliminate duplicate copies of spec in memory (e.g. $allowed_tags, $DEFAULT_ARGS['amp_allowed_tags'], and $args['amp_allowed_tags']).

Relates to #3080 and #4570.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[jwold]

#5585

Knowledge path admin umbrella. Want a little widget in the dashboard. Would like a little widget with info about latest news about AMP plugin and a set of links to twitter, amp blog, plugin site, YouTube channel.

[westonruter]

Dependency for #1401.

[westonruter]

Resolved by ampproject/amp-toolbox-php#100