Merge pull request #23 from matrix-org/media_repository

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 ``"local_content"`` directory.
+Thumbnails are stored under a ``"local_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"``
+Remote content is cached under ``"remote_content"`` directory. Each item of
+remote content is assigned a local "``filesystem_id``" to ensure that the
+directory structure ``"remote_content/server_name/aa/bb/ccccccccdddddddddddd"``
+is appropriate. Thumbnails for remote content are stored under
+``"remote_thumbnails/server_name/..."``