Get rid of the reversion package

We haven't been using it properly, and it's filled the database with old versions of items, that we don't currently use.

After moving the database to postgresql, I get lots of errors when saving items. I'm not inclined to debug the package, so I think just remove it and come up with a simpler system for storing checkpoints.

No files or tests are named. Start by locating the reversion package integration and reproducing the PostgreSQL save errors; then scope the simpler checkpoint storage and verify that items save without accumulating unused historical versions.

Written by the indexing model from the issue text.