Update user stories, delete personas

More focus on using data to make decisions – we want to move away from
presenting data just because people would find it ‘interesting’
More focus on user needs
More detail – individual metrics and modules
More user groups – ministers and senior civil servants, and members of
the public
Personas deleted because they were patronising

Still to come: links to how we meet each need.
Convert to RST from Markdown
Remove unused duplicate label

Prefer :doc: links for this need.

See http://sphinx-doc.org/markup/inline.html#cross-referencing-documents
Merge pull request #34 from alphagov/new-user-stories

Update user stories, delete personas