Maps - performance tune-up (phase 1)

See this bug for details: https://bugzilla.mozilla.org/show_bug.cgi?id=985158#c5

We show a subset of glows (high-download ones) to begin with and allow the user to switch to all glows if desired. As discussed, we'll explore replacing the animation with canvas next; I think there's value in having this in master for now.

This PR also includes some, but not all (yet), of the code cleanup suggestions by alex gibson.