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.