Removes needless ivar caching (a Rails association handles that).

Inserts a reload and a nil assign, since the association proxy doesn't
seem to that it's been destroyed through `purge`.

Thanks to rails/rails#29854! This does mean that we now depend on rails/rails master.

We need to have stable urls for blobs and variants or caching won't work. So provide a controller that can give that and redirect to the service URL upon lookup.

We don't need to lookup multiple different classes, so no need to use a globalid.

No need for this proliferation

Otherwise it can't be used to display variants. It's better anyway since all other services won't know about blobs either. Better simulation. Closes #71

Follow engine conventions more closely

Since they're only ever generated in signed form. Users never have direct access to dictate transformations.

Move all controller tests to controllers dir

On-demand variants