Add endpoints to get invited users

We want to display flash messages in admin when invites have been
cancelled. This message needs to display the user's email address, so
this commit adds endpoints to GET a single invited service user and org
user so that we can look up the email address of a cancelled user.

tests/app/organisation/test_invite_rest.py

@@ -116,6 +116,29 @@ def test_get_invited_users_by_service_with_no_invites(admin_request, sample_orga
     assert len(json_resp['data']) == 0
 
 
+def test_get_invited_user_by_organisation(admin_request, sample_invited_org_user):
+    json_resp = admin_request.get(
+        'organisation_invite.get_invited_org_user_by_organisation',
+        organisation_id=sample_invited_org_user.organisation.id,
+        invited_org_user_id=sample_invited_org_user.id
+    )
+    assert json_resp['data']['email_address'] == sample_invited_org_user.email_address
+
+
+def test_get_invited_user_by_organisation_when_user_does_not_belong_to_the_org(
+    admin_request,
+    sample_invited_org_user,
+    fake_uuid,
+):
+    json_resp = admin_request.get(
+        'organisation_invite.get_invited_org_user_by_organisation',
+        organisation_id=fake_uuid,
+        invited_org_user_id=sample_invited_org_user.id,
+        _expected_status=404
+    )
+    assert json_resp['result'] == 'error'
+
+
 def test_update_org_invited_user_set_status_to_cancelled(admin_request, sample_invited_org_user):
     data = {'status': 'cancelled'}