Update dates and deadlines design mockup

[JonellaCulmer]

What we're after: Following the usability testing conducted at the San Diego conference, we need to make design updates to the page to support those lessons learned.

Prototype: https://invis.io/SZUE03ARF4U
Test script: https://docs.google.com/document/d/1siUUEWI7InFn-rI11Rgiqrs7XFLYEdJad-CeTISS9Pk/edit
User test notes: https://docs.google.com/document/d/1fgDX_n13BMIMawSEPe5D6Oz7suj0utW3T8_Fr035_6Q/edit#
Takeaways: fecgov/fec-testing#125 (comment)

Completion criteria:

• Mock up changes to the dates and deadlines page
• Discuss with content team and information division
• Open front-end implementation ticket