Claim bot: /claim and /unclaim on issues (#1584)

Comment /claim on an issue to be assigned and get the in progress label; /unclaim releases it. A daily sweep frees claims with no PR after 72 h or with a PR idle for 14 days.

.github/workflows/ci.yml

@@ -125,6 +125,8 @@ jobs:
         run: cargo xtask web-offline-test
       - name: Affected-package selection self-test
         run: python3 -I scripts/ci/test_affected.py
+      - name: Claim bot tests
+        run: node --test scripts/claim/*.test.js
 
   clippy:
     name: Native Linux Clippy