NeverEndingReddit inbox icon - use /static/ images instead of stylesheet; use colorblind-friendly icon

reddit's spritesheet's URL appears to change periodically (due to CSS changes, presumably). Because the  Never-Ending Reddit inbox icon, `#NREMail`, sourced an old URL for the reddit spritesheet, the icon has not been displaying. To fix this issue and prevent it from reoccuring in the future,  I set `#NREMail` to use the static images. 

I also set up `#NREMail` to use the color-blind friendly icon  (again?).  As part of this fix, RES now adds a `.res-colorblind` class to the body, in case subreddit moderators want to take advantage of it.

[Relevant bug report](http://www.reddit.com/r/RESissues/comments/14b3ym/bug_never_ending_orangered_envelope_is_invisible/)