Allows all SCIM identities to be returned, paginated.

SCIM clients will sometimes send a request without a filter, using
pagination to limit results instead. Previously we would raise an error
when a filter wasn't provided, meaning we were unable to process those
requests.

Our initial approach was an MVC for Azure support, which required filter
by id. Filters are optional however, and we need to conform to the spec
more closely for Okta comaptibility.

See: https://tools.ietf.org/html/rfc7644#section-3.4.2.2