Refactor media modules. (#15146)

* Removes the `v1` directory from `test.rest.media.v1`.
* Moves the non-REST code from `synapse.rest.media.v1` to `synapse.media`.
* Flatten the `v1` directory from `synapse.rest.media`,  but leave compatiblity
  with 3rd party media repositories and spam checkers.

synapse/rest/media/download_resource.py

@@ -0,0 +1,75 @@
+# Copyright 2014-2016 OpenMarket Ltd
+# Copyright 2020-2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import logging
+from typing import TYPE_CHECKING
+
+from synapse.http.server import (
+    DirectServeJsonResource,
+    set_corp_headers,
+    set_cors_headers,
+)
+from synapse.http.servlet import parse_boolean
+from synapse.http.site import SynapseRequest
+from synapse.media._base import parse_media_id, respond_404
+
+if TYPE_CHECKING:
+    from synapse.media.media_repository import MediaRepository
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class DownloadResource(DirectServeJsonResource):
+    isLeaf = True
+
+    def __init__(self, hs: "HomeServer", media_repo: "MediaRepository"):
+        super().__init__()
+        self.media_repo = media_repo
+        self.server_name = hs.hostname
+
+    async def _async_render_GET(self, request: SynapseRequest) -> None:
+        set_cors_headers(request)
+        set_corp_headers(request)
+        request.setHeader(
+            b"Content-Security-Policy",
+            b"sandbox;"
+            b" default-src 'none';"
+            b" script-src 'none';"
+            b" plugin-types application/pdf;"
+            b" style-src 'unsafe-inline';"
+            b" media-src 'self';"
+            b" object-src 'self';",
+        )
+        # Limited non-standard form of CSP for IE11
+        request.setHeader(b"X-Content-Security-Policy", b"sandbox;")
+        request.setHeader(
+            b"Referrer-Policy",
+            b"no-referrer",
+        )
+        server_name, media_id, name = parse_media_id(request)
+        if server_name == self.server_name:
+            await self.media_repo.get_local_media(request, media_id, name)
+        else:
+            allow_remote = parse_boolean(request, "allow_remote", default=True)
+            if not allow_remote:
+                logger.info(
+                    "Rejecting request for remote media %s/%s due to allow_remote",
+                    server_name,
+                    media_id,
+                )
+                respond_404(request)
+                return
+
+            await self.media_repo.get_remote_media(request, server_name, media_id, name)