Remove uses of screen_icon() in network admin screens

Due to the recent style changes in WordPress 3.8, screen_icon doesn't appear anyway.
Add network row actions to the list table items

Adds placeholders for Edit, Dashboard, Visit, and Delete
Fill in URLs for dashboard and visit in the network row actions
Remove delete from the row actions

Let's not imply the ability to quick delete a network. That could have serious consequences any way we approach it.

We may be able to include delete instructions in the edit screen instead.
Add a (currently non-existent) URL for editing a network to the row action