Fix checkmarks to render correctly

Problem

Right now a list of checkmarks in GitHub-flavored markdown, that normally render like this (on GitHub):

- [ ] Check item 1
- [ ] Check item 2

…instead render like • [x].

Although we don’t want to make these into working check items on our end (yet, at least), we do want to render them correctly.

We can do this by doing string parsing in the Ember app itself, rather than in the markdown rendering step on the API.

[joshsmith]

Rendered client-side, although indices of their placement inside the HTML (maybe? or the markdown?) was generated in the API.

[begedin]

Just to remind you, mentions have been rendered client-side for us. They are just disabled at the moment, but they did work at some point.