VSD - hide the Edit button on custom pages

Problem

The Edit button present for custom VSDs:

Related to this comment.

Proposal

We will need to check the route slug for value_streams_dashboard (default VSD) or value_streams (YAML configured VSD), if the route is neither of these, then the edit button should follow any existing rules.

• Add new feature flag enable_vsd_visual_editor
• Add a new computed property showVSDEditControls to customizable_dashboard.vue
  • The property should return false if the feature flag is disabled and the route slug is value_streams_dashboard or value_streams
  • The property should return true if the feature flag is enabled and the route slug is value_streams_dashboard or value_streams
• Update the editingEnabled property to return this.dashboard.userDefined || this.showVSDEditControls;
• Update related tests