Add activity log

[timobrembeck]

Motivation

The default Django admin backend stores metadata about all changes made in the activity log, which makes it easier to track down specific issues and makes it more transparent which user made which changes.

Proposed Solution

• Add the same logging model as Django admin (but without installing the app in non-debug-mode, so copy + paste the LogEntry model instead of importing it)
• Create a new log entry whenever objects are created, changed or deleted via LogEntry.objects.log_action()
• Show the log of the most current actions of each region in the dashboard

Alternatives

Additional Context

This is a more general approach than #649

[svenseeberg]

I'm wondering if this feature will actually be useful (used). We know ourselves that we rarely use logs. I think we should think of specific use cases and create functionality that covers those. Example: e-mail notification used in #316