Refactor ineq cards

[srfoster]

The inequality cards rule system has a pretty cool theme. But unlike, say, the boolean algebra system -- that theme is embedded in the rule system's render function. This should be factored out. See any of the other rule systems for guidelines.

[JuliaB1]

Created a new file in the themes folder (emoji-inequality.rkt) to match the corresponding themes of the boolean algebra and clock arithmetic puzzle systems.