Add reminder modal to challenge step

[nguyetv]

The goal is to add small friction to the challenge process encouraging the challenger to address concerns with Newsrooms before committing to the challenge.

Design mockup

A popup modal that shows up after users click on "Submit a Challenge" - https://consensys.invisionapp.com/share/RAIZ3X2TVXM#/357415462_04_Challenge_Modal

a checkbox on challenge form that requires people to check before confirm button is activated - https://consensys.invisionapp.com/share/RAIZ3X2TVXM#/347132502_04_Challenge