Linkpreviewer Improvements
- Have LinkPreviewers stored in NeoChatConnection so that they don't have to be reloaded everytime the MessageContentModel is refreshed
- This means the link is never changed (it will be swiched for a new previewer with the new link)
- LinkPreviewers are stored by URL so they can be re-used by any event with the same URL
BUG: 484927 (because the offending code is ripped out)