Add markdown formatter

Deploying steps:

* Install extra dep: `pip install misaka`
* Explicitly send old format for all comments and messages (in mongo shell):

  ```
  > db.messages.update({}, {$set: {format:"moinmoin"}}, {multi: true})
  > db.comments.update({}, {$set: {format:"moinmoin"}}, {multi: true})
  ```