Update dashboard editing to save visualizations directly to the dashboard file

Problem to solve

Right now the dashboard editor saves all visualizations as separate files to the dashboard. This can lead to 1000s of duplicate visualization files because users aren't reusing what has already been built.

Proposed solution

Update the dashboard editor to save visualizations directly to the dashboard rather than referencing external visualization files. Example: !173552 (5cb407e4)

Implementation plan

frontend - 1️⃣

1. Update app/assets/javascripts/vue_shared/components/customizable_dashboard/utils.js#L114 to use the full visualization config, not just the slug.
2. Update specs to verify the change.