Refactor reactions

Currently we effectively create the reactions list in EventHandler then pass that data into a model. This reworks the model so that we just pass in a room and an event and it grabs it's own data. This means that:
- the functions in event handler are no longer required
- the model can update itself to add/remove reactions so no need to handle that in MessageEventModel
- MessageEventModel only needs to create new ReactionModels or remove old ones when no reactions exist anymore

A basic test suite has also been created for the ReactionModel

autotests/data/test-reactionmodel-extra-sync.json

@@ -0,0 +1,44 @@
+{
+  "timeline": {
+    "events": [
+      {
+        "content": {
+            "m.relates_to": {
+            "event_id": "$153456789:example.org",
+            "key": "👍",
+            "rel_type": "m.annotation"
+            }
+        },
+        "origin_server_ts": 1690322545183,
+        "room_id": "!jEsUZKDJdhlrceRyVU:example.org",
+        "sender": "@bob:example.org",
+        "type": "m.reaction",
+        "unsigned": {
+            "age": 390159121
+        },
+        "event_id": "$163456790:example.org",
+        "age": 390159121
+      },
+      {
+        "content": {
+            "m.relates_to": {
+            "event_id": "$153456789:example.org",
+            "key": "😆",
+            "rel_type": "m.annotation"
+            }
+        },
+        "origin_server_ts": 1690322545184,
+        "room_id": "!jEsUZKDJdhlrceRyVU:example.org",
+        "sender": "@bob:example.org",
+        "type": "m.reaction",
+        "unsigned": {
+            "age": 390159122
+        },
+        "event_id": "$163456791:example.org",
+        "age": 390159122
+      }
+    ],
+    "limited": true,
+    "prev_batch": "t34-23535_0_0"
+  }
+}