Set up automatic publishing to github pages

[dsd]

When a change is pushed to master, the game should auto-build for web and publish to GitHub pages.
Similar to https://github.com/GameNova-Studio/path-to-the-throne

[wjt]

#2 doesn't work: you can't get past the title screen.

Looking in the web console I see:

USER ERROR: Failed to instantiate scene state of "", node count is 0. Make sure the PackedScene resource is valid.
    at: instantiate (scene/resources/packed_scene.cpp:142)
USER ERROR: Parameter "p_child" is null.
    at: add_child (scene/main/node.cpp:1566)
```

[wjt]

The issue above is:

• Fix scanning for levels in exported game #24

I believe it will be fixed by:

• Fix loading levels in exported game #25

[wjt]

It is indeed fixed.

https://endlessm.github.io/everlasting-candy/