add new W3C Manifest so Marketplace can be added to homescreens

read https://bugzilla.mozilla.org/show_bug.cgi?id=1084548#c0

I also added other tags to support other platforms' legacy "Add to Homescreen."
Btw, are we using server.html? I see it linked to in the nginx conf, but I don't know if it's referring to src/server.html. If so we might want to modify that too