[WIP] feat: offline error page

[jessicaschilling]

About

This PR adds a more friendly and useful error page when either user's custom gateway is offline, or user tries to load a DNSLink-enabled site from an unavailable remote host.

Closes #806.

To do:

• Remove some unneeded resources in this landing page's directory
• Move text into i18n keys once agreed upon
• Wire up buttons, error-check condition, and display for window.location.hash

[jessicaschilling]

@lidel - No need to look at code for this yet, just wanted to confirm this direction with you before moving the text into i18n keys. Note that the explanatory text is intended to change depending on error context, so a wired-up version of this would only display one text-plus-button(s) combo (either "Your custom gateway ..." or "You tried to load ..."). Do you mind a quick look? Thanks 😊

[lidel]

The direction looks great, love the icon, thank you 🙏

Sidenote: this screen will be useful not only for recovering when local gateway is down. We will have two types of errors:

• Local gateway is offline
• Remote server error
  • if DNSLink is present and original server is down, Companion is able to recover by loading DNSLink website from the local gateway (example below).

[jessicaschilling]

@lidel -- I think this is ready for you to wire up at your leisure. CI is failing due to missing manifest.json but looks like that's intentionally excluded? Let me know if there's anything else I can do.