git sync and other enhancements

- Replaced Nginx with Tornado in Docker image. Resulted in smaller image size (Tornado was already installed for IJulia)
- Implemented file upload/download and directory browsing with Tornado. (better looking and more customizable)
- Simplified login screen by removing option to re-create instance. the functionality is now handled through admin and auto cleanup.
- Added sync tab with ability to synchronize git repositories on to JuliaBox account
- Enhanced admin/account tab to display SSH public key (for git authentication), JuliaBox version, and allowing user to upgrade JuliaBox image.