Implement GitHub authentication

[kostmo]

Closes #1847.

Demo

Production

https://swarmgame.net/list-games.html

Local testing

tournament/scripts/demo/server-native.sh

and

scripts/test/run-tests.sh swarm:test:tournament-host

Authentication flow

1. Users are represented by a GitHub username (primary key) and an "authentication cookie" in the SQLite database.
2. Site prompts user to login when the client's cookie is nonexistent or does not match any user in the database.
3. GitHub flow:
   1. Clicking the "Login" link redirects user to the GitHub login page.
   2. GitHub sends a code to our callback URL.
   3. use that code to get an "access token"
   4. use the "access token" to look up the username of the person who is logging in.
   5. generate and store a new cookie in the database row for that username
   6. set the cookie value on the user's client.
4. As long as the client keeps sending the cookie value known to the server, all uploads/activity will be attributed to their GitHub username.

New features

• Login/Logout
• All uploaded content is attributed to an authenticated GitHub user
• Separate pages for scenario lists and solution lists
• Download a solution file

[byorgey]

Looks good to me!

@kostmo , when you get a chance, could you take a look at #1583 and #1851 ?