Use the new file icons from gitlab-svgs in the Repository Browser

Proposal

Add a feature flag which if enabled serves the resulting dist/file_icons.svg rather than app/assets/images/file_icons.svg and uses the new getIconForFile from this package rather than file_icon_map.js. (The code should be the same as of now, as I just rebased.)

in gitlab-svgs!709 (comment 997861560)