Store changes as json in logging table

[supertassu]

We currently store data in log_entries.action - we really shouldn't. That's a varchar(191), that is bound to cause issues in the future. Let's just store that as JSON. We don't need to search log entries from database based on that, right?

For example, modified user - queue wiki permission reload,enwiki: -tooladmin should be modified user (or edited or something similar) with rest of that as structured JSON.