Add Twitter Card support (aka Twitter previews when you tweet a Mahara Page url)

[Aaron Wells]

In order for previews to show up on twitter when you tweet a URL, the site needs to have some setup done for "Twitter Cards", which is what they call the preview thing that shows up when you click "Expand". The site admin then needs to submit the URL for the site to Twitter for validation by a human being.

We can't do much about the second bit, submitting the URL to Twitter, but we can add in all the support for the Cards. The syntax is apparently quite similar to OpenGraph (which is what Facebook uses)

Here's the Twitter Card documentation: https://dev.twitter.com/docs/cards