List creation when there's no domains created is confusing

When there aren't any domains set up, we still allow people to click the "create list" button, but it's impossible to actually complete this form without any domains.

For now, I've put in a help_text saying that the list admin hasn't created any domains if that's true so at least the would-be list creator knows who to bother, but we should probably have a nicer workflow here where people who have appropriate permissions can create a domain while creating a list, or are directed to the "create a new domain" page somehow.