swh-web/coverage: Add origin count for each referenced code provider
For each referenced code provider in the archive coverage list, count the associated number of origins and display it in the coverage widget.
As this operation takes some time (between 1 and 2 minutes to get all counts), execute it once per day and cache its results to database. The cached counts will then be served instead of executing the underlying long storage queries each time.
Depends on swh-storage!145 (closed)
Related #1463
Migrated from D1076 (view on Phabricator)