Add a /contributors endpoint

[ondrae]

Description

We want to start promoting the participation of people in the civic tech movement. One of the ways to participate to commit to civic tech projects. The CfAPI provides the master list of those projects. We are already collecting the contributors to individual projects. We should aggregate them into their own endpoint.

codeforamerica.org/api/contributors

{
  objects : [
    {
      avatar_url: "https://avatars.githubusercontent.com/u/595778?v=3",
      html_url: "https://github.com/ondrae",
      login: "ondrae",
      total_civic_tech_commits: 15,
      projects_commited_to : {
        url : "github.com/civictech/projectname",
        commits : 10
      },
      ...
    },
    ...
  ]
}

[ondrae]

Other GitHub activity to collect:

• number of issues opened
• number of comments