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