add changelogs

changelogs/client_server/newsfragments/2536.feature

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+Add cross-signing properties to the response of ``POST /keys/query``

changelogs/client_server/newsfragments/2536.new

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+``POST /keys/device_signing/upload`` and ``POST /keys/signatures/upload``

changelogs/server_server/newsfragments/2536.feature

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+Add cross-signing:
+
+- Add properties to the response of ``GET /user/keys`` and ``GET
+  /user/devices/{userId}``.
+- The ``m.device_list_update`` EDU is sent when a device gets a new signature.
+- A new ``m.signing_key_update`` EDU is sent when a user's cross-signing keys
+  are changed.