[Fixes #12847] Stop using Gravatar service for the default gravar

[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.

For all pull requests:

• Confirm you have read the contribution guidelines
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• Make sure the first PR targets the master branch, eventual backports will be managed later. This can be ignored if the PR is fixing an issue that only happens in a specific branch, but not in newer ones.

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• 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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• Commit message must be in the form "[Fixes #<issue_number>] Title of the Issue"
• PR title must be in the form "[Fixes #<issue_number>] Title of the PR"
• New unit tests have been added covering the changes, unless there is an explanation on why the tests are not necessary/implemented
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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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.97%. Comparing base (58b43b0) to head (b6417d3).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12849      +/-   ##
==========================================
- Coverage   67.97%   67.97%   -0.01%     
==========================================
  Files         977      977              
  Lines       59038    59040       +2     
  Branches     6897     6897              
==========================================
  Hits        40132    40132              
- Misses      17263    17266       +3     
+ Partials     1643     1642       -1     

[github-actions]

The backport to 4.4.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-4.4.x 4.4.x
# Navigate to the new working tree
cd .worktrees/backport-4.4.x
# Create a new branch
git switch --create backport-12849-to-4.4.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 5eaad8a8f05291c1383756e64f665e608b021666
# Push it to GitHub
git push --set-upstream origin backport-12849-to-4.4.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-4.4.x

Then, create a pull request where the base branch is 4.4.x and the compare/head branch is backport-12849-to-4.4.x.