Deploy an iD preview site as a PR workflow

[andrewharvey]

https://github.com/openstreetmap/id-tagging-schema has a neat feature where each PR will built a preview site of iD incorporating the PR changes.

While iD is not the only consumer of ELI, it would still be nice to have this feature here, so that we can test how the imagery will look and work in iD before merging it.

cc @tyrasd who built this feature for id-tagging-chema