Validate slug uniqueness also in draft charts

[lucasrodes]

Core problem

Draft charts don't get their slug validated to ensure it is unique.

Context

When publishing a chart, we need to ensure that its slug is unique, or Grapher will complain when attempting to publish the chart (see image below)

However, if we save the chart as a draft, Grapher won't complain. This may seem irrelevant, but it can cause issues when working with staging servers & chart diff.

When working on a data project with Saloni, we have a workflow like:

• I work on the data & import it into a staging server.
• I share the server details with the author.
• The author (Saloni) creates some charts with the new data and saves them as drafts.
  • Why drafts? The charts might not be finished and are just the chart's first version.
  • But if they are in a staging server, why bother? Because once we merge the PR and the charts are synced to PROD, we want to keep them as drafts.

Considering this, if we use a slug that's already in use, chart-diff will show the error:

While this is caught in chart-diff (and shouldn't go unnoticed), it would be nicer if we could catch it earlier. E.g. when the author saves the draft in Admin.

Proposed solution

Apply the same validation logic to draft charts. This means running the same checks when clicking "Save draft" than when clicking on "Publish" or "Update chart".

[marcelgerber]

What would be nice here is if a slug can be null for draft charts, and only needs to be present for when a chart is published.

But otherwise, we agree with Lucas here, and it would also bring some sanity into our own lives.