Add simple drop zone

[sanohin]

Closes #149 and #186.
Added drop zone over the editor component.

[vercel]

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[codecov]

Codecov Report

Merging #198 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #198   +/-   ##
=======================================
  Coverage   85.74%   85.74%           
=======================================
  Files          24       24           
  Lines         519      519           
  Branches      103      103           
=======================================
  Hits          445      445           
  Misses         66       66           
  Partials        8        8

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[gregberge]

Thanks you!