Premium datasets keep working when their source is down

Changes since 2.39.0.

#New

• Last-known-good copy for premium datasets. Each time a premium dataset is read in full from its source, Orion keeps a small copy of its figures. The copy holds the publisher's table, with no map shapes, and is at most 4 MB. If the source later fails, maps, downloads and the API use that copy instead of failing, and say so with the copy's date. Map loads carry from_copy in their metadata, and downloads carry an X-From-Copy header.
• Only premium datasets get a copy. Ready data and other datasets are still read from their source every time. A dataset that stops being premium loses its copy.

#Fixed

• A dataset version whose source gave no updated date now records none, instead of an empty value.