Enable to not upload artifacts

Description

Right now, artifacts are the only way to keep untracked files between stages. If the artifacts are not needed after the pipeline is completed, there is no point in uploading them to the Gitlab server.

This is mitigated with expires_in, but anyway the files are uploaded unnecessarily. Moreover, for pipelines with huge artifacts this method is not feasible: gigabytes of temporary data cannot be uploaded to the server.

Proposal

Add a artifacts:upload toggle to define artifacts that are not to be uploaded to Gitlab.

Links / references

Related issues:

• Caching between stages: #336 (closed)
• Panic when uploading big artifacts bundle to gitlab: #1599 (closed)