Web UI: subnode(s) with 8 or more interfaces should include optional Migration Network by default

[ylei-tsubame]

Add Migration Network to the default list of networks for subnodes when there are 8 or more interfaces.

[ylei-tsubame]

To get this working:

• wait for the network-interface fetch,
• optionally, wait for the host detail fetch if expecting to populate fields with existing data,
• determine whether conditions are met to add a MN input group,
• don't add the MN input group again after the user specifically changed it to another network or closes the input group

Due to the current complexity of the network form, it's advised to reduce the complexity by replacing the form validation and break apart the input groups before starting this issue.