Simplify cover image markup structure

[aduth]

This pull request seeks to simplify the markup structure of the cover image block, from a set of two nested section elements to a single section element, flattening classes, and applying the hasBackgroundDim default attribute consistently.

These changes should also resolve the "corrupt" warning shown for the first block in the Demo content screen.

Testing instructions:

Verify that Cover Image block behavior continues to work as expected, particularly dimming and fixed background, and that no warning is shown in the Demo content screen.

[mtias]

@jasmussen can you give this a look, please?

[jasmussen]

I'm having some trouble with a react dom JS error with the demo content, so I can't check right now sadly. It seems unrelated to this PR, though, as it's happening on all demo content for me.

[youknowriad]

It looks like the frontend styling is broken for this PR

before:

after:

[aduth]

@youknowriad I'm not able to reproduce the issue you're seeing. I did find that I neglected to deduplicate styles between block.scss and style.scss, updated in 6a960cb. Can you take another look?

[youknowriad]

I don't know why but it seems fixed now. Thanks

[codecov]

Codecov Report

Merging #2038 into master will not change coverage.
The diff coverage is 25%.

@@           Coverage Diff           @@
##           master    #2038   +/-   ##
=======================================
  Coverage   20.24%   20.24%           
=======================================
  Files         130      130           
  Lines        4184     4184           
  Branches      712      711    -1     
=======================================
  Hits          847      847           
- Misses       2811     2812    +1     
+ Partials      526      525    -1

Impacted Files	Coverage Δ
blocks/library/cover-image/index.js	30.43% <25%> (ø)	⬆️

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