Truncate extension descriptions

This is for #8216.

- [x] Code to toggle between original and truncated description
- [x] Visual review
- [ ] Code review
Yes, but all the other code in that same if-else-if block uses `.attr()` to access the `data-` attributes. So I guess the question of which one to use comes down to consistency with the rest of the code around it for better method.

Also, I can't see to get `.data()` to work correctly. It will not pick up the `trunc-desc` value, so the description expands but never contracts.