Add default network to Organization settings

[jbair06]

Most organizations will use 'Mainnet' in practice, but some will want to use 'Testnet' or 'Custom' as the default network. Currently, the network selected is based on last session. But for users that move from one org to another, it would be beneficial to have a default network to reduce the changes a user needs to make when switching orgs.

Add a default network for each organization. The user must also be able to change the network regardless of the selected org.