Add LogContext middleware to inject org/user id

[rndstr]

We want to inject the organizationID and userID from named url path parameters in multiple places which this middleware helps with.

Not sure about the naming... open to alternatives.

[leth]

In other places we've sent the UserID and OrgID as HTTP headers, any reason for the departure from that?

[rndstr]

@leth good point, I've seen both. Who is adding these HTTP headers? Clients? Authfe? And is there a middleware to extract it into the request context?

[leth]

Fairly sure it's authfe; though the middleware lives in the users directory: https://github.com/weaveworks/service/blob/master/users/client/middleware.go

[jml]

Sometimes in other clients directly too: https://github.com/weaveworks/cortex/blob/master/pkg/ruler/ruler.go#L223

[rndstr]

Main misconception was that there was no injection (since I didn't run it through authfe locally). Another revelation was that orgID != externalOrgID, so I'm closing this. Will also revert weaveworks/billing#180 and create a PR with minor improvements I stumbled upon.