Add --allow-fork-prs option. Default to false.

Operating on forked pull requests is dangerous if it's a public repo
because anyone can make a pull request to a public repo. We default to
false like CircleCI and TravisCI who don't allow credentials to be
available on fork PRs.

server/server_test.go

@@ -26,7 +26,7 @@ func TestNewServer(t *testing.T) {
 	Ok(t, err)
 	_, err = server.NewServer(server.Config{
 		DataDir: tmpDir,
-	})
+	}, server.FlagNames{})
 	Ok(t, err)
 }