Add the option to specify a referral health facility from SNIS-RDC

[jniles]

When a patient is admitted as a referral, we should provide a list of health facilities from which they could potentially be referred. These health facilities would come from the DHIS2/SNIS-RDC list of health facilities. The user should optionally be able to indicate in plaintext the facility, should it not exist.

We could use the DHIS2 API from https://github.com/IMA-WorldHealth/dhis2-api, but would need to extend it to cover the OrgUnits API, as it does not cover this endpoint.