[KED-2186] Hide graph when loading+switching to new pipeline

[richardwestenra]

Description

I've noticed that often, when loading the graph while changing the pipeline, it's not sufficiently obvious that the chart is still loading. This is because the sidebar updates before the graph does, which gives the impression that it's ready when it isn't yet. This change aims to prevent this by removing the old graph when loading, to make it obvious that the new graph is still being generated.

This PR supercedes #282, which was an alternative fix for the same problem, but had more unwanted side-effects.

Development notes

Resetting the store appears to only take a couple of milliseconds in testing. Removing the old graph probably takes more, but likely not too much time.

QA notes

Load multiple large pipelines asynchronously, and try switching between them. The pipeline graph now disappears when the new one is being loaded and calculated.

Checklist

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added new entries to the RELEASE.md file
• Added tests to cover my changes

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