Use function to build thumbnail url in chart grid

Description

Small MR to specify just the image filename in ChartGrid.js and then build the url in ChartGrid.vue, per @mjaenicke's suggestion. This meant uploading a placeholder image to s3.

Changes Made

How to Test

Pull changes locally and run npm run dev

Related Issues

NA

Additional Notes

NA

Merge Request Checklists

• Ensure that code changes adhere to best practices documented in README.md
• Clean the code the way Vue likes it - run npm run lint --fix
• Document which browsers the site has been tested on:
  • Desktop/laptop
    • Chrome
    • Safari
    • Edge
    • Firefox
  • Mobile device
    • Chrome
    • Safari
    • Edge
    • Firefox