analytics: implement a/b testable "feature flags" via utm_soure=self and utm_campaign

[teodesian]

Now that we have all this jazz in the log DB, we can make this happen. The idea would be to configure starman such that x/n workers express the feature, while the others behave as normal.

We can also implement trackable, but still anonymous user journeys via 'utm_source=self' and utm_content flag set to the prior request_uuid. We can use this to identify major paths through the application and identify when they break.