Add basic support for an origin blocklist
This adds a boolean field, "blocklisted", to the origin documents. When this field is true, the documents are not returned in search results.
The blocklist field is sticky for updates, meaning that another run of the swh.search journal client (or further visits of the origin) will keep the origin hidden from search results.
Related to #3224 (closed).
Test Plan
Added tests for the new behavior
Migrated from D5465 (view on Phabricator)