Automate serviceworker caching manifest generation

[silverwind]

The script at https://github.com/go-gitea/gitea/blob/master/templates/pwa/serviceworker_js.tmpl should be replaced with something that automatically generates a appropriate service worker caching manifest based on the present files in public at compile time and then use that data together with StaticUrlPrefix at runtime to serve the actual URLs.

Normally, one would use a webpack plugin for this, but webpack can not know about StaticUrlPrefix, and also does not know about vendor files so it's not directly suitable, but could maybe be integrated in the compile-time part of the process.