This makes for a more seamless-looking address bar/status bar
when using the web editor on a mobile device, either directly
in the brower or installed as a progressive web app.
This also specifies a theme color for the web editor's offline
fallback.
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.
