Preview Button: Save post before navigating to preview

[aduth]

Related: #1295

This pull request seeks to address the need for a post autosave to occur before showing a preview, as otherwise the preview will not reflect unsaved changes applied to the post.

Implementation notes:

Because popup windows can only be opened in response to user interaction, the solution here is:

1. Open a popup at about:blank as soon as user clicks button, retaining reference to popup window
2. Autosave post
3. When autosave completes, update popup window reference location to saved post link

Testing instructions:

Verify that any unsaved changes are reflected in preview.

1. Navigate to Gutenberg > New Post
2. Add a title and/or content
3. Press Preview button in header
4. Note a popup window is shown, and after post is saved, you are navigated to a preview reflecting edits from your post

Ensure unit tests pass:

npm test

Caveats:

Published post autosave is not yet implemented. See #1773 (comment).

[codecov]

Codecov Report

Merging #2193 into master will increase coverage by 0.24%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2193      +/-   ##
==========================================
+ Coverage    22.3%   22.54%   +0.24%     
==========================================
  Files         137      139       +2     
  Lines        4291     4369      +78     
  Branches      722      734      +12     
==========================================
+ Hits          957      985      +28     
- Misses       2815     2858      +43     
- Partials      519      526       +7

Impacted Files	Coverage Δ
editor/header/tools/preview-button.js	94.44% <100%> (+94.44%)	⬆️
blocks/library/button/index.js	26.66% <0%> (ø)	⬆️
editor/sidebar/table-of-contents/index.js	0% <0%> (ø)	⬆️
blocks/color-palette/index.js	0% <0%> (ø)
editor/sidebar/table-of-contents/item.js	0% <0%> (ø)
blocks/library/cover-text/index.js	32.43% <0%> (+2.43%)	⬆️
i18n/index.js	88.88% <0%> (+11.11%)	⬆️

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[youknowriad]

LGTM 👍