Update notify to expect a list of string targets instead of a single … (#3548)

* Update notify to expect a list of string targets instead of a single string

* Actually do the thing I set out to do

* Fix notify.group test to expect an array of targets

* REST platform will only use the first target in the list

* Update notify platforms to expect a list of targets

* Update notify services.yaml

tests/components/notify/test_group.py

@@ -74,7 +74,7 @@ class TestNotifyGroup(unittest.TestCase):
         data = self.events[-1].data
         assert {
             'message': 'Hello',
-            'target': 'unnamed device',
+            'target': ['unnamed device'],
             'title': 'Test notification',
             'data': {'hello': 'world', 'test': 'message'}
         } == data