Don't bother responding to client requests that have already disconnected (#8465)

This PR ports the quick fix from https://github.com/matrix-org/synapse/pull/2796 to further methods which handle media, URL preview and `/key/v2/server` requests. This prevents a harmless `ERROR` that comes up in the logs when we were unable to respond to a client request when the client had already disconnected. In this case we simply bail out if the client has already done so.

This is the 'simple fix' as suggested by https://github.com/matrix-org/synapse/issues/5304#issuecomment-574740003.

Fixes https://github.com/matrix-org/synapse/issues/6700
Fixes https://github.com/matrix-org/synapse/issues/5304