Add integration with turnitin/plagiabot/EranBot

* Add a wiki timestamp parser to copyvios/misc.py
* Refactor copyvios/turnitin.py for more sensible structure
* Update templates/index.mako to incorporate diff link/timestamp and
  make it clearer that Turnitin is revision-based checking

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