[Fixes #12361] Fix captcha field visualization in signup page

[giohappy]

Checklist

Reviewing is a process done by project maintainers, mostly on a volunteer basis. We try to keep the overhead as small as possible and appreciate if you help us to do so by completing the following items. Feel free to ask in a comment if you have troubles with any of them.

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• There is a ticket in https:/GeoNode/geonode/issues describing the issue/improvement/feature (a notable exemption is, changes not visible to end-users)
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[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 64.17%. Comparing base (e0353ec) to head (763f0c6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12392      +/-   ##
==========================================
- Coverage   64.18%   64.17%   -0.01%     
==========================================
  Files         886      886              
  Lines       53286    53289       +3     
  Branches     6585     6586       +1     
==========================================
  Hits        34200    34200              
- Misses      17571    17574       +3     
  Partials     1515     1515