Move basket event processing into basket-specific repo

[rfk]

We have a new repo that's supposed to contain "all the server-side behaviours that we eventually want basket to handle naitvely":

https://github.com/mozilla/fxa-basket-proxy

Let's move the existing basket event-sending stuff out of this repo and over to that one.

[vladikoff]

To be merged...

[rfk]

I think we should hold off this until after the train is cut; give @jrgm a change to move over to using the logic in fxa-basket-proxy before removing it from this repo.

[rfk]

Re-assigning to "quality" milestone since the only thing that remains here is removing dead code.

[rfk]

@jrgm gave the ok for this on train-48, merging