Require verified certificates for federation (MSC1711)

[matrixbot]

This issue was originally created by @anoadragon453 at matrix-org/dendrite#760.

MSC1711 states that federation connections must only be made to homeservers that have a valid X.509 (SSL) certificate. The specification states this as well, thus Dendrite must enforce it.

[matrixbot]

This comment was originally posted by @JorikSchellekens at matrix-org/dendrite#760 (comment).

Will we support this: "Servers can additionally skip the certificate validation for a given whitelist of domains or netmasks for the purposes of testing or in networks where verification is done elsewhere, such as with .onion addresses"