Cleanup abbreviation

Display the official abbreviation as returned by Whitehall's API for organisations, rather than the idiosyncratic (and incorrectly lowercase) abbreviations from redirector.

Also rename the existing abbr field to redirector_abbr to make it clear what it is.

I'd like to stop requiring redirector_abbr for an organisation to be valid. 

@rgarner would like to have semantic organisation paths inside the app, rather than using surrogate keys (database IDs). I would imagine this to be stored in a field called "slug". The challenge would be:
  * it should be short (whitehall_slug is unique but long)
  * it should be predictable (ie you can guess the slug for an org)
  * it should be unique (abbreviations aren't always present, nor are they unique)
  * to decouple ourselves from redirector

That belongs in a separate pull request.