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?**
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