Implement support for <amp-story-page-attachment>

[amedina]

AMP Stories allow you to attach additional content to a page as explained here.

Edit:
Based on the research and discussion on #2782 we should do the following:

• Create a custom block for Page Attachment.
• Replicate the looks of FE in the editor.
• Selecting the Page Attachment block in the editor would open it (the same way as in FE) and reveal the inner blocks.
• Add block settings for title, CTA text, theme.
• Allow just one inner block for now: post embed (search input, will store the ID)
• The embed will display either full content in cite or some selected data, e.g. title, featured image, excerpt.

[spacedmonkey]

Steps to test.

1. Users should be able to add an attachment block. (Only one per page).
2. User should be able to change text from 'Swipe up'
3. User should be able to search for existing pages / posts via there title in autocomplete input. This input should be accessible via keyboard.
4. User should be able to remove a post / page via button in top corner.
5. User should not be able to duplicate attachment or copy two attachment blocks on one page.
6. User should be able to test a title.

[csossi]

• Changed "Swipe Up" copy to "Click Here":
  

On previewing, it still has "Swipe Up":

[csossi]

Verified in QA

[swissspidy]

Screencast: https://cloudup.com/cWMBzAeg8gh