Components: Extract Reusable PostPublishWithDropdown component

[youknowriad]

Extract a reusable PostPublishWithDropdown component to the editor/components folder.
Needed to be able to separate the edit-post and editor module

Expect some similar PRs today, I'm going to merge them as soon as the tests pass, they consist of moving some files around.

Testing instructions

• Click the publish dropdown component, it should open as expected.

[codecov]

Codecov Report

Merging #3642 into master will increase coverage by 0.06%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3642      +/-   ##
==========================================
+ Coverage   36.81%   36.87%   +0.06%     
==========================================
  Files         271      271              
  Lines        6647     6636      -11     
  Branches     1202     1202              
==========================================
  Hits         2447     2447              
+ Misses       3547     3536      -11     
  Partials      653      653

Impacted Files	Coverage Δ
editor/components/post-publish-dropdown/index.js	0% <ø> (ø)
...tor/components/post-publish-with-dropdown/index.js	0% <0%> (ø)
editor/edit-post/header/index.js	0% <0%> (ø)	⬆️
editor/edit-post/header/header-toolbar/index.js	0% <0%> (ø)	⬆️

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