Rebased attempt at hyperlinking mentions inline.

This is just a rebased and squished version of #899.

Once this is accepted, I will need to file bugs for the following:

* Right now we don't ensure that there are only spaces/punctuation around the mention, meaning e.g. our code would think `hi@toolness.com` contains a mention of `@toolness`. However, I *think* the `ExtractsMentionsFromMarkdown` class has the same limitation (not sure though).
* A class `mention` is given to the hyperlink but it's not actually given any special display via CSS.
* The list of mentions are still enumerated after the comment, but should be removed as per the discussion in #877.
* We shouldn't hyperlink mentions if the mentioned user doesn't actually exist.
* The `ExtractsMentionsFromMarkdown` class duplicates some of the functionality in this PR, albeit by analyzing Markdown source rather than the converted HTML. It would be nice to consolidate the functionality somehow.