Quote: Fix new quote, transform from list block

[aduth]

Fixes #2343

This pull request seeks to resolve issues relating to quote initialization and list transforms.

The quote block does not define a default value but assumes it to be an array, iterating by Array#map in its save implementation. While we could add a default value to the attributes definition, the approach here instead drops the Array#map since, for all I could discern, it serves no purpose (generates identical output). Instead, the value is rendered verbatim.

Further, there were some issues with list transforms, particularly for lists which do not contain any special formatting or list nesting. In these cases, list items would be plain strings, but the block's internal toBrDelimitedContent utility was not prepared to handle these cases.

Lastly, the List -> Quote transform was not transforming into the expected value type of quote and should have been wrapped in the expected array of paragraphs.

Testing instructions:

Verify that you can create a new Quote block.

Verify that you can convert from List block to Quote (and Paragraph) block with:

• No content
• Single list item (with and without formatted content)
• Multiple list items (with and without formatted content)
• Nested list items (with and without formatted content)

[aduth]

Oh failing tests, you illuminate why we'd done the map, though really shouldn't be necessary facebook/react#7038.

[codecov]

Codecov Report

Merging #2349 into master will increase coverage by 0.01%.
The diff coverage is 20%.

@@            Coverage Diff            @@
##           master   #2349      +/-   ##
=========================================
+ Coverage   24.69%   24.7%   +0.01%     
=========================================
  Files         152     152              
  Lines        4738    4962     +224     
  Branches      799     849      +50     
=========================================
+ Hits         1170    1226      +56     
- Misses       3014    3136     +122     
- Partials      554     600      +46

Impacted Files	Coverage Δ
blocks/library/list/index.js	6.55% <0%> (-0.68%)	⬇️
blocks/library/quote/index.js	14.28% <100%> (ø)	⬆️
blocks/library/separator/index.js	42.85% <0%> (-7.15%)	⬇️
blocks/library/cover-text/index.js	29.41% <0%> (-5.59%)	⬇️
blocks/library/heading/index.js	18.91% <0%> (-4.9%)	⬇️
blocks/library/latest-posts/index.js	7.22% <0%> (-2.78%)	⬇️
blocks/library/gallery/index.js	23.8% <0%> (-1.2%)	⬇️
blocks/library/image/index.js	13.95% <0%> (+1.7%)	⬆️
blocks/library/embed/index.js	49.37% <0%> (+3.92%)	⬆️

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