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.
Quite a few tests are failing. I'm wondering if this is the cause? For example: `AccountDeleter has all user association keys accounted for`

    Diff:
       @@ -13,7 +13,6 @@
         :invited_by,
         :notifications,
         :person,
       - :profile,
         :reports,
         :services,
         :tag_followings,
       
     # ./spec/lib/account_deleter_spec.rb:175:in `block (2 levels) in <top (required)>'

So the tests need a little check through, whether things are really "breaking" or just tests need patching.
Also, if you could clean the commit message and add a Changelog entry - I think this would be ready for merging.