Some functions retrieve a large amount of data to filter it later in the view. This is often the case with Datagrepper because the API does not allow complex filtering. As a result, the cache for this data does not need to be invalidated when a filter changes. This branch adds a variable so widgets can declare that.

There's also a commit allowing a cached function to declare what results should not be cached. For example if an HTTP request fails, it does not make sense to cache the empty result. This branch allows cached function to declare which values should not be cached (for example, None)