"irc://" scheme should be optional for addresses in server list dialog

When adding a new item to a server list, the dialog requests a 'Server URI', and gives an error about hostnames being required if the user enters something like "irc.quakenet.org" (it's treating that as a scheme, so you need an irc:// prefix to make it work).

Like other address inputs in the client, this field should default to a scheme of irc:// if needed.