Mailing Lists: should have a display name and description

[alexfarrill]

On the unsubscribe page we currently show you a formatted-for-humans version of the list name. We should continue using the list name as the shorthand reference for the list, but add some other features:

• A display name so you can show the user a list name that is properly formatted for humans
• A list description in case they need a further explanation/reminder of what the list is
• Only allow certain characters in the name field [a-zA-Z0-9-] so that it can't include characters that would need to be URI encoded since this is interacted with as an API
• Document the requirements for the name field and return helpful error messages in the right places

Ideally there'd be a UI for editing the display name and description, but for now it's ok if you have to use the db to do it