add classifier rule: no binary blobs

Fixes #5708. A negative rule on the repo's own review-classifier rules: binary
files and text that encodes them (base64, hex, PEM, byte arrays, data URIs)
are refused, test fixtures included. Hashes, lockfile pins, commit ids, short
tokens and text assets are not blobs.

.hyperhive/review-classifier.yml

@@ -31,6 +31,15 @@ classifications:
       implementation or public API.
     polarity: negative
     threshold: 75
+  - name: no-binary-blobs
+    description: >-
+      The diff adds binary data to the repository: a binary file, or text
+      that encodes one, such as base64, hex or PEM-armoured data, a long
+      byte array, or a data URI. This holds for test fixtures too. A hash,
+      a lockfile pin, a commit id or a short token is not a blob, and neither
+      is a text asset such as SVG, CSS or JSON.
+    polarity: negative
+    threshold: 70
   - name: tests-with-behaviour
     description: >-
       The decision-making logic a change adds or alters in Rust or