Write the upload portion of version 1 of the media repository

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+Media Repository
+================
+
+The media repository is where attachments and avatar photos are stored.
+It stores attachment content and thumbnails for media uploaded by local users.
+It caches attachment content and thumbnails for media uploaded by remote users.
+
+Storage
+-------
+
+Each item of media is assigned a ``media_id`` when it is uploaded.
+The ``media_id`` is a randomly chosen, URL safe 24 character string.
+Metadata such as the MIME type, upload time and length are stored in the
+sqlite3 database indexed by ``media_id``.
+Content is stored on the filesystem under a "content" directory. Thumbnails are
+stored under a "thumbnails" directory.
+The item with ``media_id`` ``"aabbccccccccdddddddddddd"`` is stored under
+``"local/content/aa/bb/ccccccccdddddddddddd"``. Its thumbnail with width
+``128`` and height ``96`` and type ``"image/jpeg"`` is stored under
+``"local/thumbnails/aa/bb/ccccccccdddddddddddd/128-96-image-jpeg"``