use grunt sed to flip image names for rtl

Move the last of the RTL out of the core files.

The grunt sed is a bit clunky, really I just want to pass an array
of string pairs and have them replaced without a) having to list
each as a separate task, and b) without worrying about regex
escapes for characters like [[this]].

I'll file a bug upstream and see if he's interested.

In the meantime it lets me escape some weird interaction where
javascript was adding classnames to change the image but the old
RTL solution was adding .dir-rtl and messing up the specificity.