Bug 1013783 - Shared styles missing 2x icons

Some 2x and 1.5x icons that were missing in the shared style area, ones that show up in email.

While the @2x icons may not needed for the Flame, the @1.5x icons are, and these are all part of the efforts for the general email 2.0 visual refresh tracked in [bug 950208](https://bugzilla.mozilla.org/show_bug.cgi?id=950208). With these icon additions, email will look good in 1, 1.5 and 2x targets when the other email-specific visual redesign changesets land.

tools/png_recompress.sh was used on the images before committing them to this pull request.