[WIP] amp inline gallery captions

[sparhami]

This PR is a WIP, but is not in a fully working state yet. There are a few things that need to be fixed in addition to code clean up. The initial state of this PR can be seen at https://amp-polish.firebaseapp.com/manual/amp-inline-gallery/captions.html

• Make lightboxing of slides work correctly
• Make the "See more" button work correctly, replace text with an icon
• Allow customization of the "See more" icon / message
• Change lightbox, amp-truncate-text to display the non-truncated content when going to lightbox mode

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