This:
1. Removes the `filestore` property from the `ModuleSystem` in favor of
the `runtime.resources_fs` property.
In the original code, `filestore` is equal to
`DescriptorSystem.runtime.resources_fs`. It's safe to replace it with
`ModuleSystem.runtime.resources_fs` because both runtimes are combined
using the `CachingDescriptorSystem`. It provides the `resources_fs` property
that uses the same file storage.
2. Renames `filestore` argument to `resources_fs` in the `LoncapaSystem`
constructor.
3. Adds the deprecated `filestore` property to the `ModuleSystemShim`
and `RuntimeShim`.
16fa97dde4