[webapp] Clean up and complete avatar uploading

[tomlynchRNA]

Known TODOs (non exhaustive)

• Add avatar uploading to any documents e.g. tool that don't have one yet
• Update the avartaruploader component to better handle an error in the upload API response
• Add a way to remove the avatar (suggestion: "x" button chip/badge in the "edit" forms)
• Add any missing APIs e.g. delete asset, as currently there is only an API to create one
• When user uploads new avatar, delete old one (it already overwrites but currently leaves an orphan/dangling file in gcs)