Update default avatar image

[mattiagiupponi]

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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The following are required only for core and extension modules (they are welcomed, but not required, for contrib modules):

• There is a ticket in https://github.com/GeoNode/geonode/issues describing the issue/improvement/feature (a notable exemption is, changes not visible to end-users)
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Submitting the PR does not require you to check all items, but by the time it gets merged, they should be either satisfied or inapplicable.

[giohappy]

Let's change the default avatar to a common "account icon" rather then Gravatar

[codecov]

Codecov Report

Merging #11676 (d0acf28) into master (e7b53a5) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #11676   +/-   ##
=======================================
  Coverage   62.95%   62.95%           
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  Files         875      875           
  Lines       52548    52548           
  Branches     6605     6605           
=======================================
  Hits        33079    33079           
  Misses      17946    17946           
  Partials     1523     1523