feat: make tileboard installable as web apk

[akloeckner]

fixes #819

This

• adds a dummy service worker with a fetch handler
• increases the favicon size to 512px and adds a vector source graphic
• adds a start_url to the webmanifest

The service worker's scope is automatically just the path to its js file. (See Google's primer on it.)

The ingress mode of the addon can disable the service worker via window.IN_INGRESS = true. See PR there: resoai/TileBoard-addon#32.

[rchl]

FYI: I've acknowledged this PR but didn't have time to look into it yet.

[akloeckner]

Thanks for the feedback! We're in no hurry, things are working swiftly already: I have just kept my once installed app from my test branch. ;-)

[rchl]

Made minor changes:

• optimized SVG
• declared SVG favicon in the manifest and added 256x256 size also
• small style changes

[akloeckner]

The manifest changes haven't made it to the PR yet.

[akloeckner]

Yeay! Thanks.

I believe start_url is a requirement, though. I just did a quick test on my mobile and didn't get the "install" option. I'll check again on a laptop as soon as I find time.

[rchl]

I've misinterpreted my local changes after all this time and thought that this was something that I've added when experimenting...
Should definitively restore it then.

[akloeckner]

I verified that start_url is needed to show the install button on my Windows Chrome. I opened #838 to fix this.