web app: missing static images when running locally

When running locally the web app as documented in the getting started doc , several static images are missing, as shown in this screenshot:

To provide a nicer experience to people who are starting to hack on Software Heritage, it would be nice if we can make those work.

A setting that makes serving static images in an inefficient way for development purposes only would be totally acceptable.

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Migrated from T1197 (view on Phabricator)