Fixed #20488 -- Added copy and move methods to the storage API

* also implemented these methods in the default FileSystemStorage
* fix issue with storage tests: each test from FileStorageTests was run twice (when inheriting from a TestCase class all inherited test_ methods are ran in the subclass as well)
Need `.. versionadded:: 1.8` annotations and release notes for these changes.
Also add backtickets around parameter names.