Get project avatars from KG/core, maintain on core

[ciyer]

From #2337

• Avatars: we should have support soon for that on renku core. The pattern would be similar to the one described above. Verify it's feasible and consider creating a follow-up issue.

[lorenzo-cavazzi]

This should follow a similar pattern to what we do with avatars for datasets: use POST cache.upload_file and then set it through POST project.edit
Currently, renku-core doesn't support setting an image. This will be implemented soon on their end (renku-python reference issues still missing -- to be added)
https://dev.renku.ch/swagger/#/project/post_renku_project_edit