Add button to reveal riven challenge from mods screen

[thetdw2000]

I appreciate the new system where rivens no longer take up capacity if their challenge is not yet revealed but it gets annoying having to manually equip each mod in the loadout screen just to reveal the challenge. I should just be able to reveal the challenge from the mods screen so that I can then decide if I want to do the challenge, trade it, or use a cipher.