Simplify partial rendering when the partial is named after the model

[gravitystorm]

During review of #2207 I realised that we can simplify a bunch of our partial calls. For example, if @trace is a Trace model, then instead of:

<%= render :partial => "trace", :object => @trace %>

... you can instead call...

<%= render @trace %>

... and Rails will find the partial named after the model, and pass the object into the partial automatically. It also works with collections.