Prefix all icons with mf-
All icons are now prefixed with mf- automatically in Icon.
This can be overriden by a glyphPrefix options.
In MobileUI this prefix is not configurable and happens automatically
since MobileUI should not be used outside MobileFrontend.
Since icons are rendered on special pages or inside JS
they are not subject to caching so no fallback icons using
the old css selectors are needed.
The following icons are added to MobileFrontend. Previously they
lived in Minerva which doesn't make sense given they are used here.
A follow up change (I93264024f4915fc910c792b1905b89cdc6b8b546)
removes them from Minerva
The arrow icon which lived in Minerva is moved to MobileFrontend.
The gray turns from #54595d to #a2a9b1
mobile.overlay.images is renamed to mobile.startup.images.variants
and expanded to include new icons.
the cleanup icon is removed from mobile.issues.images - it has not
been used in 2 years (since Id4b2508a2f71922a077f55e7f297f56ac7768a59)