Transfer js code in listeners.html into separate .js file

Fixes #1226 (<=== Add issue number here)

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

[ ] PR is descriptively titled 📑 and links the original issue above 🔗
[ ] tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR -- or run tests locally with grunt test
[ ] code is in uniquely-named feature branch and has no merge conflicts 📁
[ ] screenshots/GIFs are attached 📎 in case of UI updates
[ ] @mention the original creator of the issue in a comment below for help or for a review

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!

publiclab / Leaflet.DistortableImage