Some tweaks needed here:
* These are all using the same cache key (side-open), which I don't think is right.
* The doc string in """ is copy pasted, from the comments one, please update each one
* The comment in each says that they are invalidated when a comment is posted. This sounds like the correct behaviour but I don't think that's been implemented. See the following for points where you may need to invalidate the cache:
    * https://github.com/tricycle/lesswrong/blob/master/r2/r2/models/link.py#L682
    * https://github.com/tricycle/lesswrong/blob/master/r2/r2/models/link.py#L1118