Don't alias users except for one-time sync

This is part of what's necessary to make aptible/dashboard.aptible.com#234 work.

I won't try to re-explain the [Mixpanel docs](https://mixpanel.com/docs/integration-libraries/using-mixpanel-alias) on the subject, and conceptually it's a bit complicated, but basically: multiple aliases can make different streams of client-side and server-side events permanently unlinkable.

The best rule of thumb I have right now is: only alias once, from an anonymous client-side identity to a permanent server-side identity, as soon as we're able. As long as we stick to this, we should be good.