"Changes" table for sources app

[pgulley]

As a first step in our imminent collections overhaul, we want to implement a 'changes' table, that just appends records for each CRUD event in the collections application. @rahulbot has suggested a record like: "user_id, timestamp, category: str, action: str, details: str". Ideal would be to have a decorator or single line of code we can slip into the endpoints that just writes a record for every event. Ideally we make no new views for this, just add an admin table so we can monitor changes as they occur.

Tasks:

1. Agree on a schema
2. A little research on the ideal integration pattern
3. Create the table and integration point
4. Register admin app.

I'll drop in thoughts on point 2 shortly. If we have agreement on the schema I can break ground on this tomorrow.

[pgulley]

Quick note on 2- a pattern I've found for integration is to override the perform_create (etc) methods in the REST viewsets, that makes all the CRUD stuff easy to capture. Additional thought is needed for integrating e.g. rescrape attempts.
A question: Do we want to log only user-initated events? Should things that get triggered by a background task not get logged, or get differentiated from user events?

[pgulley]

This is happening in #1184 !

[philbudne]

@pgulley done?