Correct dimensions for preview overlay

During the side-by-side refactoring, those were removed,
rendering the overlay invisible, which lead to the "blocked"
state not being shown at all.
Only show preview switch if states are available

We've removed all states from RedirectorPage in order to disable the redirect.
BUGFIX Hard fallback to en_US.js in Requirements

Ideally we'll have region-less locales like en.js as fallbacks,
but for now this prevents the UI becoming non-operational
e.g. when both i18n::set_locale() and i18n::default_locale()
are set to 'en_NZ'.