model: Fetch realm emoji using events

Fetching emoji using events ensures that realm emoji updates are not missed.
Initial state of realm emoji is updated from initial_data.
Added key 'realm_emoji' to initial_data to fix key error while running pytest.
Added new tests for the handler of emoji update event.

tests/model/test_model.py

@@ -1887,3 +1887,35 @@ def test_process_realm_emoji_data(self, mocker, model,
         fetched_custom_emojis = model.process_realm_emoji_data()
 
         assert fetched_custom_emojis == custom_emojis
+
+    @pytest.mark.parametrize('response', [
+        ({
+            "last_event_id": -1,
+            "msg": "",
+            "queue_id": "1517975029:0",
+            "realm_emoji": {
+                "100": {
+                    "id": "100",
+                    "name": "urwid",
+                    "deactivated": True
+                },
+                "1": {
+                    "id": "1",
+                    "name": "green_tick",
+                    "deactivated": False
+                }
+            },
+            "result": "success",
+            "zulip_feature_level": 2,
+            "zulip_version": "2.1.0"
+        })
+    ])
+    def test__handle_update_emoji_event(self, mocker, model,
+                                        response):
+        model._handle_update_emoji_event(response)
+        # For an already active emoji which has been deactivated
+        with pytest.raises(KeyError):
+            model.active_emoji_data['urwid']
+        # For a newly created custom emoji.
+        new_added_emoji = model.active_emoji_data['green_tick']['code']
+        assert new_added_emoji == response['realm_emoji']['1']['id']