Revert tesseract.js to tesseract.js@2.0.0-alpha.16 (#14262)

The OCR feature in master is broken, there are several reasons for this:
- https://github.com/naptha/tesseract.js/issues/369
- https://github.com/naptha/tesseract.js/issues/346
- and the fact that the worker JS file is served with an unversioned URL,
  and, in the nginx config we provide, served with
  "public, max-age=31536000, immutable", leading to version mismatches as
  the old version gets cached

package.json

@@ -157,7 +157,7 @@
     "stringz": "^2.1.0",
     "substring-trie": "^1.0.2",
     "terser-webpack-plugin": "^3.0.6",
-    "tesseract.js": "^2.1.1",
+    "tesseract.js": "2.0.0-alpha.16",
     "throng": "^4.0.0",
     "tiny-queue": "^0.2.1",
     "uuid": "^8.2.0",