Add GBA support

We shipped a preliminary support for emulating GBA games, powered by a wasm build of the awesome mGBA emulator by @endrift.

The GBA page now shows games coming from the GBA database repository.

Here is "uCity Advance" running on mGBA on Homebrew Hub.

Thanks to @Dimkar3000 and @josepharhar for their examples in setting up mgba in a browser.