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.

src/eventhandler.h

@@ -231,16 +231,6 @@ public:
      */
     QVariantMap getMediaInfo() const;
 
-    /**
-     * @brief Return a LinkPreviewer object for the event.
-     *
-     * A nullptr will be returned for any event that doesn't have any links so the
-     * return should be null checked and an empty LinkPreviewer provided if null.
-     *
-     * @sa LinkPreviewer
-     */
-    QSharedPointer<LinkPreviewer> getLinkPreviewer() const;
-
     /**
      * @brief Whether the event is a reply to another in the timeline.
      */