User attribution for local instances

Created by: attfarhan

We want to be able to maintain source attribution data when a user runs a local Sourcegraph instance. In particular, we want to read from the sourcegraphSourceUrl cookie to know a converted user's first ever Sourcegraph-related URL visited. We also want to read the sourcegraphAnonymousUid cookie to associate a user with prior actions on Cloud.

See RFC 296 engineering for details.