…ds (Google Cloud Storage) and filesystem-less deployments (Appengine)

The ``thumbnail`` tag doesn't work on Google Appengine (GAE) without
these changes -- in that environment, the file system is mounted
read-only, and as such generated thumbnail images cannot be written to
it, (even temporarily).

Moreover, the pre-existing check for already-generated thumbnails
assumed a FileSystemStorage backend, (and indeed went around it); as
such, thumbnails were regenerated upon every request. These changes
leverage the file storage backend for this check, for more generalized
and optimized compatibility.

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To note, the thumbnail function is also made to rely more completely on
the default storage backend when *saving* -- refactoring this block
slightly -- to simply use ``default_storage.open()``. This features
compatibility with both remote and local storage backends, without
additional work.

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Finally, this change set updates the AUTHORS file, (in line with
contribution guidelines).

cannot presume even that default_storage.open() and
TemporaryFile/SpooledTemporaryFile will be enough for PIL image.save;
rather, necessary to write thumbnail image to memory, first.