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-frontend/src/app/button.element.scss

@@ -1,8 +1,8 @@
 :host {
 	display: inline-flex;
 	align-items: center;
-	gap: 0.5rem;
-	padding: 10px 20px;
+	gap: 0.75rem;
+	padding: 0.5rem 1rem;
 	border-radius: 0.5rem;
 
 	font: {
@@ -10,14 +10,14 @@
 		size: 15px;
 		weight: 500;
 	}
-	line-height: 1;
+	line-height: 1.5;
 
 	pointer-events: auto;
 	cursor: pointer;
 	user-select: none;
 
 	background: #0006;
-	color: #FFF;
+	color: #FFFC;
 	backdrop-filter: blur(16px);
 	transition: {
 		property: background, color;
@@ -28,6 +28,7 @@
 
 :host(:hover) {
 	background: #0008;
+	color: #FFFF;
 }
 
 .icon {