add GET/POST rest endpoints for whitelist

GET /<service_id>/whitelist
    returns all whitelisted contacts for a service, separated into two lists

POST /<service_id>/whitelist
    removes all existing whitelisted contacts, and replaces them with the
    provided new entries

(todo: dao work + tests)

app/dao/service_whitelist_dao.py

@@ -0,0 +1,11 @@
+from sqlalchemy import or_
+
+from app import db
+from app.models import ServiceWhitelist
+
+def dao_fetch_service_whitelist(service_id):
+    return ServiceWhitelist.query().filter(ServiceWhitelist.service_id == service_id).all()
+
+
+def dao_add_whitelisted_contact(obj):
+    db.session.add(obj)