Implementation of /similarity REST endpoint

I think this is called enough that it can't be allocating tens of megabytes of data structure per call. Yes it's desirable to keep the model generic, so I like to think of methods like "top cosine similarity" rather than "most similar" as if they may be reused in other contexts. Here it is a performance concern that probably has to drive some specialized method support into the model. Put it this way -- `/recommend` is already slower than it needs to be and it has this optimization.