Add config and checks for trusted ID servers (#206)

* Add config for trusted ID servers

* Add new error

* Implement check for trusted ID server

* Complete unfinished comment

* Make comment more explicit in the config file

* Use go standard errors in membership.go

* Use standard errors instead of JSON responses in threepid

* Doc errors

* Remove unused parameter

dendrite-config.yaml

@@ -12,6 +12,12 @@ matrix:
     private_key: "/etc/dendrite/matrix_key.pem"
     # The x509 certificates used by the federation listeners for this server
     federation_certificates: ["/etc/dendrite/server.pem"]
+    # The list of identity servers trusted to verify third party identifiers by this server.
+    # Defaults to no trusted servers.
+    trusted_third_party_id_servers:
+      - vector.im
+      - matrix.org
+      - riot.im
 
 # The media repository config
 media: