Storyteller.ai API integration (#49)

This PR integrates asset retrieval from the Storyteller.ai back-end API,
and begins implementing long-term scene saving and storage.

### Media retrieval
Instead of fetching raw asset URLs, going forward, media files from
remote sources (e.g., in URL parameters) should be specified with a path
like the following:
```
remote://<token>.<filetype>
```
where
* `token` is a media token (e.g., "m_70b77neq9jsgmy5v31mekzchcbd223")
* `filetype` is a file extension appropriate for the type of file being
  fetched (e.g., "gltf")

The `remote` protocol will be recognized by a [custom asset
reader](https://github.com/storytold/storyteller-bevy/blob/70726db111f6ccafd47feb055c5546540fef1786/studio/src/remote/reader.rs)
which will retrieve the file by calling a Storyteller API endpoint.

### Scene storage
Additionally, this PR begins laying the groundwork for
storing/retrieving Bevy scenes to/from the backend instead of solely
in-memory. This work is still incomplete -- as of this PR, a button has
been added to the web UI allowing the user to download the scene
file locally for testing/debugging purposes.

### Test URL
Media retrieval from the backend can be tested by [visiting this
URL](https://deploy-preview-49--storyteller-studio.netlify.app/?mode=viewer&mixamo=remote://m_70b77neq9jsgmy5v31mekzchcbd223.gltf)
in Chrome with the `--disable-web-security` flag (to bypass CORS errors)

studio/src/lib.rs

@@ -28,6 +28,7 @@ mod interaction;
 pub mod math;
 mod mixamo;
 mod persistance;
+mod remote;
 mod scene;
 pub mod screenshot;
 mod skybox;
@@ -46,6 +47,7 @@ pub use crate::{
     math::InterpolationPlugin,
     mixamo::{MixamoImportEvent, MixamoPlugin},
     persistance::PersistancePlugin,
+    remote::RemoteAssetPlugin,
     scene::{LoadSceneEvent, ScenePlugin},
     skybox::{SkyboxId, SkyboxPlugin},
     ui::UiLayerPlugin,
@@ -251,6 +253,7 @@ pub fn start_internal(
         #[cfg(feature = "wasm")]
         viewport::wasm::ViewportPlugin,
         WebAssetPlugin,
+        RemoteAssetPlugin,
         DefaultPlugins.set(WindowPlugin {
             primary_window: Some(Window {
                 resolution: viewport_size.into(),