Add "Icons Allowed" selection to design challenges

[trevorgerring]

I've updated the design policies, see #409 With that I've split Icons out into its own policy. I would like to add a separate selection for "Icons Allowed" along with the current selection for "Stock Art Allowed". We want to be able to select these independently.

[trevorgerring]

@ajefts Any updates here?

[ajefts]

This one is likely going to get deferred until we expand the "payments app" to become the new challenge management app. Likely more of a focus next quarter. We're avoiding spending cycles and $ on Direct so we can get to a better place.