Components: Move all the Post Editor's layout components under a "post-edit" folder

[youknowriad]

closes #2761

Now that we moved all the reusable editor components under the "components" folder, this PR moves the remaining components corresponding to all the components building the "post-edit" page layout to a folder named "post-edit".

Later, I think it makes sense to move all these components to a separate "post-edit" module which will require to split the Redux store to two stores: one for the layout and one for the editor data. We can also decide to avoid using a "redux" store for the layout and just stick with local state or more "modular" state approaches.

Testing instructions

• Just make sure Gutenberg loads and works.

[mtias]

Thinking of calling it edit-post instead. Once we have page templates / theme editor it would be edit-theme or edit-template and they would sit next to each other in the directory list.

[gziolo]

I can confirm that it still works. editor folder looks really nice with those changes introduced 👍

[codecov]

Codecov Report

Merging #3608 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #3608   +/-   ##
=======================================
  Coverage   36.95%   36.95%           
=======================================
  Files         268      268           
  Lines        6673     6673           
  Branches     1202     1202           
=======================================
  Hits         2466     2466           
  Misses       3555     3555           
  Partials      652      652

Impacted Files	Coverage Δ
...tor/edit-post/sidebar/post-pending-status/index.js	0% <ø> (ø)
editor/edit-post/sidebar/post-schedule/index.js	50% <ø> (ø)
editor/edit-post/sidebar/post-author/index.js	0% <ø> (ø)
editor/edit-post/sidebar/post-sticky/index.js	0% <ø> (ø)
editor/edit-post/header/publish-dropdown/index.js	0% <ø> (ø)
.../components/block-list/block-contextual-toolbar.js	0% <ø> (ø)	⬆️
editor/edit-post/sidebar/post-visibility/index.js	40% <ø> (ø)
editor/edit-post/modes/visual-editor/inserter.js	93.33% <ø> (ø)
editor/components/navigable-toolbar/index.js	5.88% <ø> (ø)
editor/edit-post/sidebar/page-attributes/index.js	0% <ø> (ø)
... and 25 more

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