Allow CDN to make the module work out of box

Currently using modules with 3rd party libraries often require extra steps to set up, and for some users this can be difficult.

While I understand the benefits of hosting the library locally, I think that it would improve general user experience if the module could work out of the box via CDN, while still providing the option to host it locally.

This could easily be done at dropzonejs_library_info_alter, if the library files are not found in the system then it could fall back to the CDN urls.

This would also help modules that depend on the DropzoneJS module (for example Simple Media Bulk Upload) as those modules would work out of the box as well, instead of having the issues of installing for example that upload module and finding out that they need to actually set up DropzoneJS module while that was only pulled in as a dependency.

So the modification here would be easy to do, but I am not sure if the maintainers would welcome such a change.

We could of course do this inside the Simple Media Bulk Upload module, but such a modification should really be in the DropzoneJS module.