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-validplainlink-event.json

@@ -0,0 +1,14 @@
+{
+    "content": {
+        "body": "https://kde.org",
+        "msgtype": "m.text"
+    },
+    "event_id": "$validlink1:example.org",
+    "origin_server_ts": 1432735824654,
+    "room_id": "!test:example.org",
+    "sender": "@example:example.org",
+    "type": "m.room.message",
+    "unsigned": {
+        "age": 1234
+    }
+}