Pleroma is using the wrong timezone to create flake ID's

Probably same cause as #1671. After moving from a server using UTC time (in Nevada) to a server using CEST time (in Germany), no new posts were added to profiles, timelines, etc, because they were being given flake ID's with a lower value than the highest ones that already existed. Changing the server to use UTC time did not resolve the issue - but after waiting for a few hours, the generated ID's were high enough to make the issue non-apparent. This should probably be resolved by always using UTC to generate ID's.