Re-use create file functionality for "Add README" button

[fiskus]

• Changelog entry

[codecov]

Codecov Report

Merging #3173 (d55e2c0) into master (49ca588) will increase coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3173      +/-   ##
==========================================
+ Coverage   34.98%   35.00%   +0.02%     
==========================================
  Files         661      661              
  Lines       29007    28985      -22     
  Branches     4263     4263              
==========================================
  Hits        10147    10147              
+ Misses      17672    17650      -22     
  Partials     1188     1188              

Flag	Coverage Δ
api-python	90.72% <ø> (ø)
catalog	8.29% <0.00%> (+<0.01%)	⬆️
lambda	86.05% <ø> (ø)

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Impacted Files	Coverage Δ
catalog/app/containers/Bucket/Summary.js	0.00% <0.00%> (ø)

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