Add audit log to the admin panel

[m4tx]

The admin panel should have a sort of "audit log" - each action done in the panel should be saved to the database, and then we could display a list of recent actions in the homepage, or a history of a given model instance.

An open question is how to define a data model to store the actions. The baseline is probably the following:

• There is a well known set of possible actions (create object, modify object, delete an object, etc.)
• For each action we want to know who did it and when
• We want to be able to filter the object ID, so we can display a history for it
• We need to store which columns were edited in case of the "modify object" action, so we can display them as well