The `start_url` in the PWA manifest.json must be relative for it to
work in subfolders (like in the official Web Editor page).
(cherry picked from commit 85c1a41021)
This allows to install it as an app, and provide offline support (after
the first run).
Practically, this boils down to adding a JSON file as a manifest, an
offline page to be displayed when the cached files are not avaialble,
and a JS file to cache resources and return them.
The reason for the "first run requirements" is that some browsers, will
emit an "install" by just visiting the page (to see if the JS code is
compatibile), and we do not want to force casual visitors to just
download the 10 MiB+ compressed editor WebAssembly file without pressing
the start button.
Special thanks to Hugo Locurcio (Calinou) for the initial work.
