Stop hardcoding trust of old matrix.org key (#5374)

There are a few changes going on here:

* We make checking the signature on a key server response optional: if no
  verify_keys are specified, we trust to TLS to validate the connection.

* We change the default config so that it does not require responses to be
  signed by the old key.

* We replace the old 'perspectives' config with 'trusted_key_servers', which
  is also formatted slightly differently.

* We emit a warning to the logs every time we trust a key server response
  signed by the old key.

docs/sample_config.yaml

@@ -952,12 +952,43 @@ signing_key_path: "CONFDIR/SERVERNAME.signing.key"
 
 # The trusted servers to download signing keys from.
 #
-#perspectives:
-#  servers:
-#    "matrix.org":
-#      verify_keys:
-#        "ed25519:auto":
-#          key: "Noi6WqcDj0QmPxCNQqgezwTlBKrfqehY1u2FyWP9uYw"
+# When we need to fetch a signing key, each server is tried in parallel.
+#
+# Normally, the connection to the key server is validated via TLS certificates.
+# Additional security can be provided by configuring a `verify key`, which
+# will make synapse check that the response is signed by that key.
+#
+# This setting supercedes an older setting named `perspectives`. The old format
+# is still supported for backwards-compatibility, but it is deprecated.
+#
+# Options for each entry in the list include:
+#
+#    server_name: the name of the server. required.
+#
+#    verify_keys: an optional map from key id to base64-encoded public key.
+#       If specified, we will check that the response is signed by at least
+#       one of the given keys.
+#
+#    accept_keys_insecurely: a boolean. Normally, if `verify_keys` is unset,
+#       and federation_verify_certificates is not `true`, synapse will refuse
+#       to start, because this would allow anyone who can spoof DNS responses
+#       to masquerade as the trusted key server. If you know what you are doing
+#       and are sure that your network environment provides a secure connection
+#       to the key server, you can set this to `true` to override this
+#       behaviour.
+#
+# An example configuration might look like:
+#
+#trusted_key_servers:
+#  - server_name: "my_trusted_server.example.com"
+#    verify_keys:
+#      "ed25519:auto": "abcdefghijklmnopqrstuvwxyzabcdefghijklmopqr"
+#  - server_name: "my_other_trusted_server.example.com"
+#
+# The default configuration is:
+#
+#trusted_key_servers:
+#  - server_name: "matrix.org"
 
 
 # Enable SAML2 for registration and login. Uses pysaml2.