Add commentable `body` to comments export

**Is your feature request related to a problem? Please describe.** Comments exports are hard to navigate because a content id or url doesn't give context. **Describe the solution you'd like** I'd like to add a column with the `commentable_body` so that one can have context in which the comment was made. **Describe alternatives you've considered** Via the API **Does this issue could impact on users private data?** No **Funded by**

OSP