Proposal + comments export

Is your feature request related to a problem? Please describe.
As a participatory space admin and with proposal component active, I want to export proposals + comments in one single actions, not having to export them in two actions and having as a result two different documents. Also, it is a problem because now you have these two documents with comments not related with proposals (only by "commentable_id" in comments export).

Describe the solution you'd like
From admin panel, proposal component, I would like to have the button "export all > proposals + comments" as a CSV, JSON and Excel.

Describe alternatives you've considered
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Additional context
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Does this issue could impact on users private data?
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