Campaign Finder - when I make a selection within a category, all other options should be unselectable

[mikefantini]

For instance, if in cause I select Animals, all other causes should be unselectable.

We should also add some sort of x in a circle so it's clear to the user that they can remove their selection (need to make sure this treatment doesn't get too chunky on mobile).

cc: @julielorch @crystalruthbell @barryclark

[DFurnes]

Fixed. Let's revisit the styling at standup tomorrow morning.