codimd: build with nodejs-8_x

This diff regenerates the package sets for `codimd` and `codemirror`
using NodeJS 8 to get rid of the deprecated[1] `nodejs-6_x`.

Additionally the following issues had to be fixed during the update:

* The package `js-sequence-diagram` has been removed from the NPM
  registry and was replaced by a security holding package[2]. The
  package was published by a third-party (upstream only supports bower
  builds), so it's unclear whether the package will re-appear[3].

  As the tarballs still exist (and the hash didn't change), the package
  will be loaded manually into the build env.

* For the babel-related packages, `dontNpmInstall` will be set for
  `node2nix` installs as some of those packages bundle a
  `package-lock.json` that triggers `ENOTCACHED` errors for optional
  dependencies[4].

For now it should be sufficient to use NodeJS 8 (`codimd` v1.2.x doesn't
support NodeJS 10), in the long term we probably want to use `yarn2nix`
here with NodeJS 10. This is much rather a fix to get rid of another
NodeJS 6 dependency.

[1] `nodejs-6_x` is about to be deprecated, see #58976
[2] https://www.npmjs.com/package/js-sequence-diagrams,
    https://github.com/npm/security-holder
[3] https://github.com/bramp/js-sequence-diagrams/issues/212
[4] https://github.com/svanderburg/node2nix/issues/134

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