fix: #9127, scope service worker to relative_path for the forum

[oplik0]

Adds a Service-Worker-Allowed header set to forum url (nconf.get('relative_path')) on assets/src/service-worker.js URL and uses scope option during its registration to ensure the service worker is correctly scoped to the entire forum and only the forum.

This is the implementation of what I described in #9127 (comment) - sorry it took so long, had some other stuff to do and I was actually kinda afraid I'll waste my time as someone will do it a bit faster than me (well, I had it happen here once with a pretty simple fix that I wasn't fast enough to make into a PR before one of the maintainers managed it and here I basically described all changes :)

Should fix #9127 and this issue was also mentioned in #9193. I found it thanks to this thread on the community forum

I can confirm that Chrome now asks user to install the page (on the desktop it's in the right part of the address bar) with these changes, when it didn't do so before. Haven't tested with other/mobile browsers though, but I'd be very surprised if it was different.

[barisusakli]

Thanks @oplik0

[barisusakli]

@oplik0 I pushed these changes to https://community.nodebb.org/ and started getting the following message.

ServiceWorker registration failed:  DOMException: Failed to register a ServiceWorker 
for scope ('https://community.nodebb.org/') with script ('https://community.nodebb.org/assets/src/service-worker.js'): 
The path of the provided scope ('/') is not under the max scope allowed ('/assets/src/'). 
Adjust the scope, move the Service Worker script, or use the Service-Worker-Allowed HTTP header to allow the scope.

Not sure why the Service-Worker-Allowed header isn't working

[pitaj]

Could it not be forwarded by nginx?

[oplik0]

I forgot that in some installations /asstes/* paths will be served by reverse proxy, so setting a cookie for it in NodeBB won't work.

I'm not sure where to move the route though - maybe going back to just relative_path/service-worker.js (so just /service-worker.js in routes/meta.js) would work fine?

[barisusakli]

Yeah I think that's how it was initially before getting moved to /assets/src.