Add `RemixProject` mutation

[patch0]

We need a mutation that allows an existing project to be remixed.

mutation($id: ID!) {
  remixProject(id: $id) {
    project {
      id
    }
  }
}

ID might be identifier, although ID would be better, what with the whole locale thing happening.

[create-issue-branch]

Branch issues/152-Add_RemixProject_mutation created!