rather than try reconstruct the results object, better to guard against the xrange step argument being 0

synapse 500s on a call to publicRooms in the case where the number of public rooms is zero, the specific cause is due to xrange trying to use a step value of zero, but if the total room number really is zero then it makes sense to just bail and save the extra processing

Matthew's fixes to the unit tests

Add federation_domain_whitelist

gives a way to restrict which domains your HS is allowed to federate with.
useful mainly for gracefully preventing a private but internet-connected HS from trying to federate to the wider public Matrix network

Extracted from https://github.com/matrix-org/synapse/pull/2820

add registrations_require_3pid and allow_local_3pids

Use the right path for url_preview thumbnails

This was introduced by #2627: we were overwriting the original media for url
previews with the thumbnails :/

(fixes https://github.com/vector-im/riot-web/issues/6012, hopefully)

 * [ ] split config options into allowed_local_3pids and registrations_require_3pid
 * [ ] simplify and comment logic for picking registration flows
 * [ ] fix docstring and move check_3pid_allowed into a new util module
 * [ ] use check_3pid_allowed everywhere

@erikjohnston PTAL

lets homeservers specify a whitelist for 3PIDs that users are allowed to associate with.
Typically useful for stopping people from registering with non-work emails

Make storage providers configurable

Add decent impl of a FileConsumer

Fix bugs in block metrics

... which I introduced in #2785

better exception logging in callbackmetrics

when we fail to render a metric, give a clue as to which metric it was