Fixes #264 - Add per-replay comment user uniqueness constraint

With this applied, commenting multiple times on a replay will produce an
error instead of accepting the >1 comments.

The full error message the user gets is "Adding your comment failed: User - You may only comment once on a replay." Some minor hackery can apparently be applied to fully customise the message, but since rails doesn't have the option by default, I haven't tried.

From a UX standpoint, the add comment box should ideally be hidden after making a comment, and/or if you already have a comment posted. I haven't implemented this yet.