Allow to save / load code to/from gist

[certik]

Currently we can load gist if you know the url scheme (so it's not easy if you don't know or remember it), and we can save by copy & pasting into the gist manually.

To improve this, let's add "save" and "load" buttons or menu items.

• The "load" button will create a pop up window which asks you to put in a full github url of a gist (e.g. https://gist.github.com/certik/9fc75ebf47019f75994ef7e251402218), and it will parse it and automatically redirect to the proper dev.lfortran.org url with the "gist" appended (e.g. https://dev.lfortran.org/?gist=certik/9fc75ebf47019f75994ef7e251402218).
• The "save" button would ideally open the gist and put the code in, either via github url (if it allows it), or by github API, or at least copy the code to a clipboard, and tell the user to go to gist and paste the code there.

To make this smooth, we can (optionally!) allow the user to set a github token, that will be saved locally.

Related: #115