Components: Extract a reusable PostFeaturedImage component

[youknowriad]

Related to #2761 (comment)

All these (the sidebar panels) must be refactored slightly to drop all the "layout" pieces and keep only the "forms". For example, the sidebar panels like PostTaxonomies, we should extract only the PostTaxonomyForms into a reusable component without the expandable panel behavior that is specific to our current UI. (This step could be done in a separate PR preceding the directory structure change)

This PR addresses the comment above for the PostFeaturedImage component.

Testing instructions

• Test that the last revision link is working as expected

[codecov]

Codecov Report

Merging #3291 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #3291      +/-   ##
=========================================
- Coverage   31.05%     31%   -0.05%     
=========================================
  Files         232     233       +1     
  Lines        6521    6531      +10     
  Branches     1163    1164       +1     
=========================================
  Hits         2025    2025              
- Misses       3770    3779       +9     
- Partials      726     727       +1

Impacted Files	Coverage Δ
editor/post-featured-image/index.js	0% <0%> (ø)
editor/sidebar/featured-image/index.js	0% <0%> (ø)	⬆️

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