Error on configuration page when aka_host is empty

Problem/Motivation

There's an error on the configuration page when the "Additional host" setting has no hosts listed.
Also, the name is misleading since you'll need to include the host name of the site if you want the Tidy process to update it. It's more like "All host names to treat as being the same as this site"

Steps to reproduce

The default configuration doesn't include any hosts in the list, so this happens on install :(
If you've already added at least one host name Load a configuration with an empty aka_host setting.

Proposed resolution

The configuration page should not result in an error when aka_host is empty. There are two parts to update

• Include a 'web' host name in the default install (base_uri is 'https://web/', so it's a good example/hint)
• Fix the error on the setting page

User interface changes

Updated the text on the setting page and in the schema.yml.

Test

If you install Link Magician with the updated branch there will be an example in the default configuration. Remove the host 'web', save the configuration and reload.