Create upload release artifacts

Description

Refactor of process to create release tarball, Singularity and Docker images. No longer needs intermediate files, Makefile steps simplified.

CI updated to automatically generate release artifacts on git tag, with tarball and Singularity image uploaded to digitalocean storage. In case of CI/gitlab/digitalocean issues, the artifacts can also be generated locally.

Related issues

Notes to review (code/docs/QA)

This is based on !498 (merged), so that should be approved / merged first

Tests

General

• Tests have been added that prove my fix is effective or that my feature works
• Related tests have been modified/removed

Hypothesis testing:

• Soak testing has been done
• Distribution between positive / negative cases has been checked

Database

• Includes changes to database schema
• Includes necessary database migrations

Configuration

• Includes changes to configuration
• Includes configuration migration instructions in documentation

Merge checklist

• Self-review of code has been performed.
• Feature review and validation against specification has been performed (if applicable). Apply label: QAdone
• Need for documentation has been evaluated and, if necessary, updated. Apply label: docsdone
• Code and implementation has been reviewed by other core developer (including any changes based on initial review). Apply label: code reviewdone