Components: Extract the WritingFlow component as a reusable Editor component

[youknowriad]

Related to #2761 (comment)

This PR extracts WritingFlow component to the reusable components folder.

Testing instructions

• Test that arrow navigation between blocks works

[codecov]

Codecov Report

Merging #3569 into master will increase coverage by 0.07%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3569      +/-   ##
==========================================
+ Coverage    35.1%   35.17%   +0.07%     
==========================================
  Files         265      265              
  Lines        6734     6723      -11     
  Branches     1220     1218       -2     
==========================================
+ Hits         2364     2365       +1     
+ Misses       3693     3683      -10     
+ Partials      677      675       -2

Impacted Files	Coverage Δ
editor/components/writing-flow/index.js	1.47% <ø> (ø)
editor/modes/visual-editor/index.js	0% <0%> (ø)	⬆️

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[gziolo]

I'm still able to move blocks, code looks good 👍