Fix loading screen for default Bevy app

[TimJentzsch]

Closes #134.

This PR adjusts the default index.html we serve with bevy run web to work better for a Bevy app that doesn't have any special configuration.

We don't assume anymore that the canvas is already created with ID #bevy, but instead watch for a new canvas to be created (which is the default behavior).

We also adjust the styling to better work with this setup and change the web background color to the default Bevy app background color.

[BD103]

Works like a charm, and fixes fun things like this:

use bevy::prelude::*;

fn main() {
    App::new().add_plugins(DefaultPlugins).run();
}

[BD103]

Merge this once you feel it's ready! :D