feat: make group icons on homepage show their group images as icons

[ChasNelson1990]

Description

We wanted the icons on the homepage that show the different resource types to be more visual. I have given each group and image_url and that is now rendered inside the little box.

Also, I made the flag a little thinner.

The zarr-ckan PR includes updated demo data loader for these icons.

relates fjelltopp/zarr-ckan#
relates (but is not dependent on) fjelltopp/ckanext-fjelltopp-theme#31

Checklist

• The Jira ticket for this issue has been updated to "Ready to Review" or equivalent.
• I have developed these changes in discussion with the appropriate project manager.
• My code follows the general Fjelltopp documentation (see Confluence).
• I have made corresponding changes to the Fjelltopp documentation (see Confluence).
• I have rebased this branch with master.
• New dependency changes have been committed.
• I have added automated tests that prove my fix is effective or that my feature works.
• New and existing tests pass locally with my changes.
• My changes generate no new warnings.
• I have performed a self-review of my own code.
• I have assigned at least one reviewer.
• I have assigned at least one label to this PR: "patch", "minor", "major".

[A-Souhei]

Looks good to me.