Log project configuration changes

[flodolo]

Right now, there's no history of what changed in a project configuration, which makes it unnecessarily hard to debug issues (the last one, an extra locale added to pre-translation for Focus Android, but we've had others).

Maybe it can be as simple as storing the data we want to track in a JSON before saving and after saving, then store the differences in a log with user and timestamp.

[mathjazz]

Let's do this as part of unifying the Pontoon and Django Admin. Django Admin already has a built-in support for logging user actions.

Note that in addition to that, we already have 3 other systems to track user actions:

1. ActionLog
2. PermissionChangelog
3. UXActionLog

At least the first two should be consolidated (#2195).