Convert image to webp

As the Google Lighthouse report suggests, we should convert all images to a next gen image format as this reduces network traffic.

created merge request !33 (closed) to address this issue

mentioned in merge request !33 (closed)

currently on hold, because converting all images to webp would require a change in the webpack.config of the invenio-assets module

added Blocked label

mentioned in merge request !43 (merged)

Looks like a leftover, the static images are already in webp format.

closed