Truncate extension descriptions

This is for #8216.

- [x] Code to toggle between original and truncated description
- [x] Visual review
- [ ] Code review
If I understand correctly, the ` toggleClass` part of the chain would read `toggleClass("expand-desc trunc-desc", showFull)`? I just tried that, and while it successfully expanded to the full description, it did not set it back to the shortened version. Checking the inspector, upon expanding the description, the link is manged by the `class` attribute being converted into a boolean attribute (read: `<a class data-extension-id="" title="" href="">...</a>`).