Code review comment for lp:~edwin-grubbs/launchpad/bug-521934-missing-sprites

Summary
-------

Re-added .branch and .cves css rules that were accidentally removed when
switching to automatic generation of the combined sprite image.

Implementation details
----------------------

Re-added the .branch css rule and the .cves css rule.
    lib/canonical/launchpad/icing/style-3-0.css.in

Added some error handling.
    lib/lp/services/spriteutils.py

New sprite image and positioning file for generating the combo.css
from the style-3-0.css.in.
    lib/canonical/launchpad/icing/icon-sprites
    lib/canonical/launchpad/icing/icon-sprites.positioning

Tests
-----

./bin/test -vv -t sprites.txt

Demo and Q/A
------------

* Open https://code.launchpad.dev/~mark/firefox/release--0.9.1/+merge/1
    * The branch icon should appear after "Proposed branch:" and "Merge into:".
* Open https://bugs.launchpad.dev/debian/+source/mozilla-firefox/+bug/3
    * The cves should float to the right.