Rename Organisation.abbr to redirector_abbr

This makes it clear that this field is a product of redirector, not something
which has more general meaning. We need to stop using this because it wont
exist for organisations that never passed through redirector and because we
should be using the official abbreviation as available in the Whitehall API.
Display the genuine abbreviation from Whitehall

Redirector's abbreviations were different (wrong) when compared with the
authoritative source (Whitehall).
Secondly, redirector abbreviations won't be available when we stop using it as
a primary data source.
Merge pull request #60 from alphagov/cleanup_abbreviation

Cleanup abbreviation