Adds 'Web Stories List' Block

[divyarajmasani]

Summary

This PR adds the 'Web Stories List' Block, the block has options to switch between the following types:

• Latest Stories: Shows recent stories based on applied filters.
• Select Stories: Shows user picked stories. Users are presented with a story picker modal.
• Story URL: Embeds story URL provided by the user.

Relevant Technical Choices

• The existing 'Web Stories' block is still present and this block uses some of its components.
• To inject blockSwitcher into the 'Web Stories' blocks' edit component we are using the editor.BlockEdit filter.

Difference between view markup choices:

• Circles / Carousel:
  -- On non-AMP pages 'glider-js' is used to render the carousel.
  -- On AMP pages it uses amp-carousel extension.

Lightbox Effect:

• On non-AMP pages, a single lightbox div is used with a single amp-story-player, adding stories as needed.
• On AMP pages, we are using multiple lightbox div with one amp-story-player for each story.

To-do

• Check if we can remove multiple amp-story-player on AMP pages for the lightbox.
• Check for any performance bottlenecks.
• Update/Simplify CSS used for the views.

User-facing changes

Adds a new 'Web Stories List' block that can embed web stories on pages/posts.

Testing Instructions

• Create/Edit any page/post.
• Look for the 'Web Stories List' block under block inserter or start typing /web and select the block.
• Choose block-type and view-type, toggle controls from the right sidebar and preview the changes.

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Fixes #78 #132 #79 #129 #131 #42 #133 #93 #38

[spacedmonkey]

This needs rebase / merge.

[divyarajmasani]

closing this in favor of other small PRs
#188 #190