Feature/json exporter

A quick first pass at a JSON exporter. Would welcome some feedback around which fields are required, useful, etc.

For #5343

Note that this hasn't touched the XML export functionality at all.
Missing attrs:

- username
- disable_mail
- show_community_spotlight_stream
- auto_follow_back
- auto_follow_back_aspect (There is only `_in` for this so it would need to fetch aspect name here!)

Also, I would remove `serialized_private_key`. IMHO we should not include that here, rather when it needs to be exported IMHO there should be a separate button to do that, maybe it could popup the text in a dialog.

Opinions?

Also, if we outscope posts and comments from this pull, those parts should obviously not be here.