Add endpoint for auto update via GH hooks.

[kdzwinel]

This creates an endpoint on the server that fetches new version of the app whenever github lets us know about changes (via a webhook). We are protecting form the abuse by using a simple password that's not stored anywhere in the code.

This follows the https://support.glitch.com/t/tutorial-how-to-auto-update-your-project-with-github/8124 tutorial.

[Charlie-belmer]

Looks pretty good! One minor thing on hash checking which isn't that important as written, but probably good to do in case one day this becomes more sensitive.