Feature/github javascript for github configuration refactor

Purpose

The current Github project configuration widget uses a mix of jQuery and mako. The mako template receives it's context from <code>AddonGitHubNodeSettings#to_json</code>, which has been deprecated.There is currently no JSON endpoint for fetching information about a project's Github addon settings. This differs from the other file storage addons. More importantly, this endpoint is necessary prerequisite if we want to render the Github configuration widget client-side

Completes milestone https://github.com/CenterForOpenScience/openscienceframework.org/milestones/Github%20configuration%20refactor

Changes

.mako files now uses Knockout data-binding to process the front-end. JSON end-points have been added & corresponding view & helper functions written.Test cases added to check the functionality of newly added view functions.