Fix tree shaking for dynamically-added class names

[westonruter]

Prevent tree shaking CSS rules that use class names which are dynamically added, including those for:

• amp-dynamic-css-classes (naturally)
• amp-carousel
• amp-form
• amp-user-notification
• amp-access/amp-access-laterpay
• amp-image-lightbox
• amp-live-list
• amp-sidebar
• amp-sticky-ad
• amp-video-docking

Fixes #1946.
Fixes #1490.

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Build for testing: amp.zip (1.1-alpha-20190313T180459Z-d5495203)

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