Unify registration between single site and network configurations

[wpmuguru]

The registration processes in 3.0 differ between single site registration and network signup. We should look at unifying the registration process. One way we could do this is

• keep the existing registration process for single sites
• in a network, users register first using the same process as single sites
• in a network, once registration is complete, direct the new user to the site creation page (which could be somewhere in the dashboard) where they could create their site

There are also several open tickets related to issues with the network signup process:

#13827
#12022
#12030
#12032

(There may be more.) We could clean up/eliminate these in process. I'd like to see this included in the discussions once we get closer to the 3.1 dev cycle.

Related: #14412

+1, by the way. Probably excellent as a 3.2 task.