Use global NPM repository

Opencast partly uses its own NPM registry for caching JavaScript
libraries used when building the project. When this was introduced, the
idea was to ensure versions would always be the same and that Opencast
could still be build when something happened to the global registry.

• The first goal, fixing versions, which was never ensured by using a
  custom registry but is now ensured using the `package-lock.json`
  files.

• The second goal, ensuring the registry exists, is currently not
  archived since multiple parts of Opencast use the global registry
  anyway. Furthermore, with the ever increasing importance of NPM, its
  highly unlikely for the global registry to just cease to exist without
  a prior notification.