Sort by name in Github.get_repositories().
Remove unused functions from Github class.

Removed functions are:
- Github.num_issues
- Github.has_issues
- Github.url_to_repo_name
Rename load_issues2() to load_issues().
Determine label text color in controller only.

Remove get_label_color_type() from Github model to avoid dealing with
presentation in model.
Move Github.API_PROXY to app/config.jsenv.

The configure name is now GITHUB_API_PROXY.
Avoid replicate API requests to get repo info.
Issue github api request only once for each url.

We cannot rely on repositories[name] to determine whether or not we
should issue github api request. Because repositories[name] is only
set when response is received asynchronously. Before the response is
received, checking repositories[name] will still fail and we will
issue duplicate api requests.