[draft] add file export and github sync

[lihebi]

Introduction

This PR implements file export and GitHub sync. Users can sign in and link their GitHub account on the profile page. Each project can be linked to a specific GitHub repo, and an "export to github" operation will export the project to files and commit to the GitHub repo.

This PR is only a draft, there're still some details left, e.g., how to maintain the git repo files when we have multiple replicas of the API servers. We'll not use GitHub sync for now.

Screenshots

login and link GitHub account:

link a project to a GitHub repo:

[forrestbao]

link a project to a GitHub repo:

it would be better if the user can select the Github repo to link to, and the repo may not be under the linked GIthub username but any organization or repo the user is authorized to. See HackMD's menu below where I authorized it to access two repos, under two user/org names.