networks: Use radio button for network modes

See: #157 (closed). The connection mode is currently a drop down with values "auto", "manual" and "shared". It is not very clear what these do. Especially how the "shared" mode works. Change this drop down into radio buttons and be more descriptive. I suggest:

• Automatic: Configure the network parameters automatically
• Manual: Configure the network using parameters provided below
• Shared: Allow other machines on this network to use FreedomBox for their Internet connectivity