Refactor LinkPreviewer

Refactor `LinkPreviewer` to take an event and put the functions for getting the link in the class itself. This means the functions in `EventHandler` are no longer required.

This mr also sets up `LinkPreviewer` so that it is automatically updated when an event is edited. This includes changing the link if edited, and it can handle a message having a previous link removed or a one added when one didn't exist before.

Also adds test suite.

autotests/data/test-linkpreviewerintial-sync.json

@@ -0,0 +1,24 @@
+{
+  "timeline": {
+    "events": [
+      {
+        "content": {
+            "body": "https://kde.org",
+            "format": "org.matrix.custom.html",
+            "formatted_body": "https://kde.org",
+            "msgtype": "m.text"
+        },
+        "origin_server_ts": 1704648567967,
+        "sender": "@example:example.org",
+        "type": "m.room.message",
+        "unsigned": {
+            "age": 112
+        },
+        "event_id": "$validlink:example.org",
+        "room_id": "!test:example.org"
+      }
+    ],
+    "limited": true,
+    "prev_batch": "t34-23535_0_0"
+  }
+}