Enhance the Contribute section

:construction_worker: 

Enhance the Contribute section of Supermarket by:

- adding a sidebar w/ chef's open source projects + current user specific info
- creating a become-a-contributor page that explains the steps to signing a CLA and why they should sign a CLA
- create an All Contributors table that lists the individuals who have signed an ICLA or CCLA

Trello card: https://trello.com/c/Uc4yJUsQ