Move to supporting multiple charts per page for Insights

What does this MR do?

1. Move to supporting multiple charts per page for Insights

• Config file change
  • No longer accepts single chart, array only
  • Display warning for old config detected
• Loads new charts upon tab change
  • Renders those successful
  • Flash message for errors
  • Summary of error instead of chart

2. Announce warning that there was a problem reading config file
3. Announce errors loading individual charts
4. Move to a dropdown instead of a tabbed page picker

• Better accommodates a large list of charts

Multi charts preview

Invalid config preview

Chart loading errors

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• Docs change required
• Replace Docs images for dropdowns required
• Change to default config required

What are the relevant issue numbers?

Closes #10720 (closed) Closes #10466 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer