Avoid breaking when entering an IPv6 address

[ExE-Boss]

Trying to preload an IPv6 address results in the web interface failing to display anything and redirecting to https://hstspreload.org/?domain=.

See also:

• https://hstspreload.org/?domain=2606:4700:4700::1111 (redirects to https://hstspreload.org/?domain=)
• https://hstspreload.org/api/v2/preloadable?domain=2606:4700:4700::1111 (the REST API works fine)
• chromium/hstspreload#105

[lgarron]

See #43 for IPv6 connectivity issues from Google Cloud (where hstspreload.org is hosted).

I believe this should be handled the same way as #105.