Grab Tesseract's trained models from GitHub, instead of from the Alpine
Linux repos. Over the past few months, the models in the Alpine Linux
repos did not remain stable, leading to CI issues.

Since the models are already pre-trained and available through
Tesseract's repo on GitHub, we can use the release tarball that they
offer to install them in the container image, which is basically what
the upstream packages are doing as well.

In order to make sure that we have no regressions, at the time of this
commit we ensured that the hashes of the models offered through the
Alpine Linux repos and the models offered from the GitHub release are
the same. Also, in order to detect future regressions or foul play, we
check the downloaded models against a known checksum. Given that these
models change every few years, updating the checksum should not be an
issue.

Fix #357

Restore the OCR languages to the state they were in
66d3c40, with some minor changes. We
can now do so because we download all the trained models, not just the
ones that Alpine Linux offers.

Remove the Kurdish (Arabic) language ("kur_ara") from the list of
languages that we offer for OCR, since it's not included in the
installed languages.

Interestingly, it is not present in the Apline Linux repos as well, so
this was probably an omission in the first place.

Test that the languages that we provide to users for OCR match the
languages that are installed in the container image

Fixes #417