Add populate_network action

[rmccue]

At the moment, populate_network_meta is the only hook available to do extra work when a new network is created. However, this is called before the initial site is created when installing the first network. This means it's impossible to do certain things on network creation, such as creating new sites.

(I want to be able to create a new site on network creation so that the network always starts with two sites: the migrated main site and another site.)

[jeremyfelt]

I've wanted this too.

There are two scenarios for populate_network(). In the most common, we're moving a single site install to multisite and applying initial configuration. In the other, it is called separately to create a new network. When a new network is created manually, the steps for assigning a main site are skipped and you're on your own to complete the setup tasks that need to run.

Where does the action make sense in this? If it makes sense for both cases, we may need to pass some contextual information. If it only makes sense for initial install, populate_network may not be the right name for the action.