Rendered entity when no bundles (file entities)

[mikemccaffrey]

@olafgrabienski in #14: File: As there are no file view modes at all, the format "Rendered entity" shouldn't be available. There is also sort of a bug: When you choose "Rendered entity" and try to configure it, you see that there is no view mode. So you hit "Cancel", but you can't return to the page, instead you get the following message: "An illegal choice has been detected. Please contact the site administrator."

If you choose a rendered entity right now when referencing file entities, there are no bundles available. We need to do a check to make sure an entity can actually be rendered as an entity before allow that display to be selected, and if there is no bundles we need to not display that option.

We should also solve the problem where files don't have bundles, but that change should probably be made in the next release of core.