Generate manifest.json when gitea web start up but not rendering when requesting

[lunny]

The route /manifest.json could be rendered when start gitea web but not always rendering when requesting. The generated file could be saved to data/html directory or object storage in future. And the router could just serve the file.
So that we could add the file to a CDN to accelerate the site.

[silverwind]

Could probably embed in directly in HTML:

<link rel="manifest" href='data:application/manifest+json;utf8,{"unesaped json": true}'/>

Some characters like # will need URL-escaping in that format. Not totally sure the single quotes will work, so maybe base64 should be more compatible (and doesn't need escaping):

<link rel="manifest" href="data:application/manifest+json;base64,eyJpY29ucyI6W3sic3JjIjoibG9nby0xOTIucG5nIiwic2l6ZXMiOiIxOTJ4MTkyIn0seyJzcmMiOiJsb2dvLTEyOC5wbmciLCJzaXplcyI6IjEyOHgxMjgifV19" />