xss: Install Form

This addresses an issue reported by Aishwarya Iyer where inserting `<img src
=x onerror = prompt(1)` into any text field on the install form will execute
in the browser after the system is installed and you log in. This is due to
us not sanitizing the content before it’s saved in the database. This adds
`Format::htmlchars()` to the installer to ensure the text field data is
sanitized properly.