Have SirCAL use the "Active" flag from the API metadata

[melange396]

Sir Complains-a-Lot lets us know about potential data pipeline problems by noticing when our indicator signals havent been properly updated for a while. Sometimes a signal may be discontinued for any number of reasons, and it will stop getting updated with new data. To prevent SirCAL from alerting on such signals, it consults a list of "retired" signals. Unfortunately, maintaining this list requires enumerating them all by hand (such as here and here) and the list will only grow larger over time.

Instead of relying [solely?] on the "retired" signal list, SirCAL can make use of the Active status in the metadata available through the Delphi Epidata API to determine if a signal is worth alerting on -- if a signal is not marked as being "Active", it is no longer expected to receive updates and it can thus be ignored.

A caveat to this is that SirCAL currently uses the covidcast client package to access metadata. That client uses the /covidcast_meta endpoint, but the "Active" status is only available from an alternate endpoint, /covidcast/meta.