Add column "immune_status" to table subjects

[philiptzou]

Enum: HIV, transplant, chemotherapy, autoimmune-disease

[philiptzou]

Further discussion determined four enum values:

• HIV
• Medical
• Unknown: the immune status is not indicated by the study
• None: the person is not immunosuppressed

The column should be created allowing null value. A null value means we haven't looked up the immune status for the subject.