FIX: @username mentions when there are multiple code blocks in a post.

Meta: [All of my internal users show as coming from 127.0.0.1!](http://meta.discourse.org/t/all-of-my-internal-users-show-as-coming-from-127-0-0-1/6607)

This fixes the regular expression used to identify @ inside `<code>` or `<pre>` blocks in the `postConversion` hook.

One drawback of this fix is that the regex has now become *really* complex.