Use the new comment model. Fixes #461

I have run a migration in production that copies all the nits into the new top-level `Comment` model, and that script can be re-run as many times as we need.

Once everything is in production I'll make a separate commit that deletes the Nit model, and we can run a data migration to delete all the duplicate nits.

@theotherzach Would you mind taking a look at this?