Add _redirects file for SPA behavior

[lidel]

If we leverage rainbow and dnslink for hosting, we need _redirects for SPA setup in case NGINX 404 logic does not engage correctly.

Does not cost us much, just include static SPA _redirects file in the build output:

Ref. https://docs.ipfs.tech/how-to/websites-on-ipfs/redirects-and-custom-404s/#examples

[SgtPooki]

What should the content of this _redirects file include? just /* /index.html 200?

this should be able to be added to public/_redirects and automatically be included in the dist folder by webpack

[lidel]

I see we already have redirect to root that preserves path (?helia-sw), so acting on that and redirecting back after SW is initialized may be better approach, ensures SW is registered for top level scope.

I've updated rule in in #53 but we need to wire it up in follow-up PR.

[lidel]

Reopening as we still need to wire up handler forthese HTTP 302 with ?helia-sw to preserve path when the first request hits the server without SW being registered yet.

Example:

http://helia-sw-gateway.localhost:3000/ipns/docs.ipfs.tech/how-to/pin-files/ redirects to http://helia-sw-gateway.localhost:3000/?helia-sw=/ipns/docs.ipfs.tech/how-to/pin-files

Rationale: ease of deployment.

I think 302 rule is slightly better than /* /index.html 200 rule because it makes deployment easier by not requiring the backend to be IPFS gateway running in deserialized mode.

HTTP 302 flow will work without backend being IPFS gateway with _redirects support (all you need is to set up nginx config to return index.html for 404 errors and that is all).

[SgtPooki]

@lidel we shouldn't need ?helia-sw anymore, a simple refresh or redirect to root should work, but if we need "something" for the redirect file to trigger, ?helia-sw will be fine (just ignored, since we don't need it)

[SgtPooki]

we have e2e first-hit tests for this now and they are successful