WIP: Gantt diagrams

What does this MR do?

This MR adds a new tab to project issues with a Gantt diagram of the project. It integrates https://gitlab.com/ganttlab/ganttlab-live; most of the rendering code and styles are still from the original project.

Are there points in the code the reviewer needs to double check?

Yes. I have no idea what I'm doing.

Why was this MR needed?

• See issue #15305 (closed).
• See https://gitlab.com/ganttlab/ganttlab-live
• See ganttlab/ganttlab-live#10 (closed)

Screenshots

• The issues below the diagram will be removed in the final version. They are there only for easier debugging.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• #15305 (closed): Gantt-like chart for visualizing milestones
• ganttlab/ganttlab-live#10 (closed)
• https://gitlab.com/gitlab-org/gitlab-ee/issues/985: Time tracking
• #4058 (closed): Relations between Issues
• #21453 (closed): Add sub-tasks (aka sub-issues) to Issues and Issue Board