Move viewer to its own repo and remove copy-to-S3 script

[aparlato]

Per #10 (comment) let's move the standalone diff viewer into its own repo: mapbox-gl-style-diff-viewer.

This will mean we can remove the copy-to-S3 Github action in this repo as well. Once the new repo is created, we can figure out the best way to continue copying the viewer into S3.

cc @ebrelsford

[ebrelsford]

The repo has been created and added ot the carto-tools team.