Add a checkbox to allow searching the EranBot/plagiabot database for
Turnitin results, and display them in a similar form to the on-wiki
EranBot reports if they exist.

Add a new module (copyvios/turnitin.py) to handle fetching and parsing
the EranBot results.

Bug: https://phabricator.wikimedia.org/T110144

TODO: tweak display HTML/CSS; refactor/clean up turnitin.py;
      improve dev set-up so it doesn't always default to testwiki
      and can test without hardcoding page title