Web UI: add a way to browse origins, other than search

It would be nice to have a way in the Web UI to browse existing origins.

It is very useful in small deployments (e.g., development deployment in docker), but would also be useful on the real archive and there it would be the equivalent of the "explore" feature that major forges offer.

As a first approximation we could just have a "dumb" pagination that is the UI equivalent of the /origins/ API endpoint. Bonus point would be to have a way to sort the returned origins in a more meaningful way, e.g., "most recently updated origin first" (but I don't know if the Web UI has easy access to the needed info for this at the moment).

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Migrated from T2605 (view on Phabricator)