elmPackages.elm-pages: init at 2.1.11
`node-packages.nix` was regenerated after having run `generate-node-packages.sh`
in order to pick up dependencies for `elm-pages`. In addition, we add `elm`,
`elm-review`, and `elm-optimize-level-2` to the `PATH` for `elm-pages` since those
are needed at runtime.
A patch is also included to fix an issue where files are copied from the `nix-store`
as read-only (credit to @bcardiff for that fix). An upstream issue
https://github.com/dillonkearns/elm-pages/issues/305 has been opened to track this.