Disable captcha registration by default in tests

fixes https://github.com/vector-im/riot-web/issues/3155

Make `prev_state` field optional

The `prev_state` field on events is not specced and so synapse shouldn't
explode if an event is missing the field.

Fixes #4787

Implement soft fail

When re-syncing device lists reset the state

Add some debug logging for device list handling

We keep track of what stream IDs we've seen so that we know what updates
we've handled or missed. If we re-sync we don't know if the updates
we've seen are included in the re-sync (there may be a race), so we
should reset the seen updates.

Document the `generate` option for the docker image.

Use static locations for Riot icons