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."
previously, with damper, I used to just open up http://localhost:8675/server.html to test how things worked with minified assets. I assume that's what it's good for.