[WIP] feat: offline error page
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 https://github.com/ipfs-shipyard/ipfs-companion/issues/806.
To do:
- [x] Remove some unneeded resources in this landing page's directory
- [x] Move text into i18n keys once agreed upon
- [ ] Wire up buttons, error-check condition, and display for
window.location.hash


@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 -- 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.