Restore BungeeCord and Velocity support

[zml2008]

This restores IP forwarding from api-7, picking up from #3076.

Main changes from that PR are:

• Use Sponge's networking API where possible
• Merge the BungeeCord and Velocity mixin plugins into one toggle
• Add a note before the vanilla offline mode warning listing out the active forwarding mode, and force offline mode when IP forwarding is enabled, since offline mode is required for IP forwarding to work.

The configuration is tweaked somewhat from that PR. Now, to configure Velocity support, the following steps are required:

1. In the modules section of sponge.conf, set ip-forwarding to true.
2. Update the ip-forwarding section of the same file to match the following:

ip-forwarding {
    # The IP forwarding mode to use with a proxy. Supported values:
    #  - NONE: Do not forward IP addresses
    #  - LEGACY: Use the BungeeCord/pre-1.13 protocol for IP forwarding (CAUTION: This protocol is insecure)
    #  - MODERN: Use the Velocity protocol for IP forwarding
    # When any forwarding mode but NONE is selected, the server will be put into offline mode and will only accept connections from proxies.
    mode=MODERN
    # The player info forwarding secret from your Velocity configuration.
    # Only used with 'MODERN' forwarding mode.
    secret="<velocity secret here>"
}

/cc @astei your review would be appreciated

[astei]

Looks good to me!