Hide custom analytics dashboard Edit button for VSDs

What does this MR do and why?

Adds new enable_vsd_visual_editor feature flag to toggle the visibility of the Edit button for custom Value stream dashboards.

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Feature off	Feature on

How to set up and validate locally

1. Enable the FF Feature.enable(:group_analytics_dashboard_dynamic_vsd)
2. Create a custom Value stream dashboard (docs)
3. View your newly created VSD
4. Verify that the Edit button appears in the top-right when enable_vsd_visual_editor is enabled, but is hidden when disabled

Related to #452638 (closed)