Extend the crawler into a url / link unfurling preview service api

[brendanheywood]

We already do basic previews of external public urls with their page title, it would be fairly simply to extend this to grab other metadata for better previews.

• favicons

• meta description

• open graph type, images and text

• twitter previews

• Previews would only be generated if a setting was on

• OR if an extra argument was passed into the crawl api (ie called by 3rd party code)

• expose an new api for other plugins to which does a synchronous ajax preview call

https://medium.com/slack-developer-blog/everything-you-ever-wanted-to-know-about-unfurling-but-were-afraid-to-ask-or-how-to-make-your-e64b4bb9254