Added thumbnails to insert step preview

[Divy123]

Fixes #618

Make sure these boxes are checked before your pull request (PR) is ready to be reviewed and merged. Thanks!

• tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR -- or run tests locally with rake test
• code is in uniquely-named feature branch and has no merge conflicts
• PR is descriptively titled
• ask @publiclab/reviewers for help, in a comment below

We're happy to help you get this ready -- don't be afraid to ask for help, and don't be discouraged if your tests fail at first!

If tests do fail, click on the red X to learn why by reading the logs.

Please be sure you've reviewed our contribution guidelines at https://publiclab.org/contributing-to-public-lab-software

Thanks!

[Divy123]

@jywarren please review.

[jywarren]

Can you share a screenshot? Thanks!!!

[Divy123]

I am sorry . There are some errors I have recently seen. I will be opening a new PR soon enough.