Add some padding between warnings in the add-on header

[willdurand]

Fixes mozilla/addons#15872

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Before:

After:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.14%. Comparing base (8d4d9dc) to head (2667b14).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #13850   +/-   ##
=======================================
  Coverage   98.14%   98.14%           
=======================================
  Files         265      265           
  Lines       10636    10636           
  Branches     3259     3259           
=======================================
  Hits        10439    10439           
  Misses        184      184           
  Partials       13       13           

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