[AndyB] Open graph thread 1.0

Uses first attachment in threads for open graph.

When creating a new post in Facebook or Twitter, the open graph function will use the first embedded attachment in your thread as the image for the new post.

How to use:

Copy thread URL from your forum. Now go to Facebook or Twitter and start a new post, paste your thread URL.

Questions and answers:

Q: What image is used if the thread doesn't have any attachments?
A: By default your logo is used. However this is not desirable. You should create an Open Graph specific logo which will be 200x200 and enter the path here:

Admin control panel -> Style properties -> Basic options -> Metadata logo URL

I suggest uploading the image to the following directory:

data/assets/logo/open_graph.png

You will need to create the assets/logo directory as XenForo does not have those directories by default.