Add Autolinker.js

Autolinking library from https://github.com/gregjacobs/Autolinker.js
Create autolink UI helper

Use Autolinker to link emails and URLs in any text.

We have to manually escape the text before linking, to ensure we don't
introduce XSS vulnerabilities etc. The Spacebars.SafeString ensures
that Spacebars won't escape the content any further by default.

We allow for autolinking text that has already been sanitised.
chatroom: autolink chats
forum: autolink forum messages