Enhance the Contribute section

:fork_and_knife: 

Enhance the Contribute section of Supermarket by adding more information about CLA signing and creating a table of all contributors.

todo:

- [x] adding a sidebar w/ chef's open source projects + current user specific info
- [x] creating a become-a-contributor page that explains the steps to signing a CLA and why they should sign a CLA
- [x] create an All Contributors table that lists the individuals who have signed an ICLA or CCLA
- [x] make new styles added responsive
- [x] make `User.all_cla_signers` less awful

Trello card: https://trello.com/c/Uc4yJUsQ
I mentioned using left joins over HipChat because I assumed we'd only want one query. However, given the contributors index template, I think the general strategy you have here is a better approach. I would like to see if we can eager load the `:icla_signatures` and `:ccla_signatures` associations, as well as the `:github_accounts` association. Otherwise we'll be issuing between 3 and 7 additional queries per contributor when rendering the view.

Happy to pair on it tomorrow :smile: