WIP: add generic_dedupe module for tracking dedupe changes.

[bcipolli]

To do:

• Unit tests

This is an alternate approach to #201 - tracking deduping, allowing manual deduping through the admin interface, and making deduping reversible.

The approach here is:

• Add a true_model_id field for any entry that can be deduped.
• When true_model_id is set, all foreign keys onto that model are automagically migrated to the "true model", and a DedupeLogEntry is added for each foreign key that was changed.
• If true_model_id is changed, then the old DedupeLogEntry values are used to revert to the original model, and the new true_model_id is applied.

I've tested this manually and it seems to work (set / reset / unset). I will work on unit tests, but I wanted to get a basic version out there for discussion first.

The code isn't huge, but it's complicated and likely incomplete. The goal here is to start with an MVP that works for our core use-cases.

The models used in this code are:

• DedupeMixin - adds the true_model_id field and provides a filtered_objects manager that gives a list of rows that are core data / not unwanted variants.
• DedupeLogEntry - extends LogEntry and keeps track of info to make dedupe reversible.

Finally, some screen shots:

Setting "true model" to an alternate city:

Dedupe log entries auto-created, showing the foreign keys set by the above manual setting.

Unset the "true model", and ...

The foreign keys are reset (not shown here), and the log entries are deleted:

[bcipolli]

I have made a develop branch, where we can merge features like these for further use and testing.