Consume icon.json files as assets

Instead of loading `icons.json` and `file_icons/file_icons.json` from
`node_modules/@gitlab/svgs/dist`, we can also make them assets and
consume them from the asset pipeline. This means that if assets are
pre-compiled inside of `compile-test-assets`, they will be included in
the asset bundle and our rspec tests can pass _without_ relying on the
`node_modules` folder.