Add a unique index to Organisation.api_id

The change of field type from text to string is necessary before
adding this index, so that the field is limited in length.
Validate presence of all fields on Organisation
WiP: Add a rake task to fetch organisations from Whitehall

This depends on the as-yet unmerged Organisations API adapter in
the gds-api-adapter gem ( in 913cee3 ).

This task fetches the current list of organisations from Whitehall
(which is the canonical source for organisations), creates new ones
in the database and updates any which have changed.