Add get_identifier methods to Person, Position and Organisation

It's useful to be able to get an identifier in a particular
identifier scheme for the objects that might have such an identifier.
This commit introduces a get_identifier method (that takes a scheme
as a parameter) to Person, Position and Organisation via a new
IdentifierMixin.
Set the classification field when creating a PopIt organization

This seems to be closest to the name of an OrganisationKind.
Correct the PopIt-specific spellings of "organization"
In current PopIt, 'role' is used rather than 'title'
Add identifier to the properties before sending to PopIt
Fix deletion of all existing records for paginated results
Update popit-django for getting results from paginated PopIt
Add a get_identifiers method to IdentifierMixin
Don't fail if there are multiple identifiers for the object

... just pick the first one, until we can disambiguate the identifiers
in org.mysociety.za.
The format of dates in PopIt has changed

Now dates are just partial ISO 8601 dates (YYYY, YYYY-MM or YYYY-MM-DD)
rather than objects including a formatted form, and so on.
Add a method to IdentifierMixin to return identifiers from all schemes
Add all known identifiers when sending data to PopIt
Also send contact details for people and organizations to PopIt
Also send alternative names to PopIt