HierarchyWidget: Icons not available

[VincentKneip]

Problem
Some icons used within HierarchyWidget are not available in our project. Example:

current situation	should be like this

The empty boxes should contain the image part.svg, which is part of the ols4 repository (see here). Some similar images like instance.svg exist as well.

Question
How are we going to deal with the problem? Can we proceed similarly as with 32px.png, including the images in our repository, but not having to include them besides the built bundle as a user of our widgets?

Replicate
Use these parameters for HierarchyWidget in the Storybook:

• api="https://www.ebi.ac.uk/ols4/api/"
• entityType="class"
• iri="http://purl.obolibrary.org/obo/UBERON_2001747"
• ontologyId="uberon"