Replace some hardcoded refs to playground.wordpress.net web app (#42)

## Motivation for the change, related issues

We are deploying a private Playground instance and want it to reference
itself rather than the original public site. In addition, these changes
will help folks who also want to self-host.

## Implementation details

In general, this PR replaces references to playground.wordpress.net with
relative URL references where those references might interfere with
Playground platform advances deployed to a private Playground instance.

There are a number of references to
`https://playground.wordpress.net/plugin-proxy.php`, and I am leaving
those alone for now. `plugin-proxy.php` requires a GITHUB_TOKEN
environment var, which requires further configuration, and this would be
particularly annoying for folks running local Playground servers. For
now, let's just keep using the public hosted version of that script and
adjust later if required.

In other cases, like that of the reference to
https://playground.wordpress.net/logger.php, I am leaving the references
as-is until there is a benefit to changing them.

## Testing Instructions (or ideally a Blueprint)

- CI

Author: brandonpayton

packages/playground/client/src/index.ts

@@ -110,7 +110,7 @@ export async function startPlaygroundWeb({
 	corsProxy,
 	shouldInstallWordPress,
 }: StartPlaygroundOptions): Promise<PlaygroundClient> {
-	assertValidRemote(remoteUrl);
+	assertLikelyCompatibleRemoteOrigin(remoteUrl);
 	allowStorageAccessByUserActivation(iframe);
 
 	remoteUrl = setQueryParams(remoteUrl, {
@@ -192,15 +192,40 @@ function allowStorageAccessByUserActivation(iframe: HTMLIFrameElement) {
 }
 
 const officialRemoteOrigin = 'https://playground.wordpress.net';
-function assertValidRemote(remoteHtmlUrl: string) {
+const validRemoteOrigins = [
+	officialRemoteOrigin,
+	// Allow hosting remote from same origin
+	location.origin,
+	'http://localhost',
+	'https://localhost',
+	'http://127.0.0.1',
+	'https://127.0.0.1',
+];
+/**
+ * Assert that the remote origin is likely compatible with this client library.
+ *
+ * Prior to this assertion, there were cases where folks used the client library
+ * from playground.wordpress.net with other origins and eventually ran into
+ * compatibility issues when the two sides went out of sync. This way,
+ * we discourage that practice which is likely to lead to breakage for the
+ * embedding app.
+ *
+ * @param remoteHtmlUrl The URL for remote.html
+ */
+function assertLikelyCompatibleRemoteOrigin(remoteHtmlUrl: string) {
 	const url = new URL(remoteHtmlUrl, officialRemoteOrigin);
-	if (
-		(url.origin === officialRemoteOrigin || url.hostname === 'localhost') &&
-		url.pathname !== '/remote.html'
-	) {
+
+	const validRemote =
+		validRemoteOrigins.includes(url.origin) &&
+		url.pathname === '/remote.html';
+
+	if (!validRemote) {
 		throw new Error(
 			`Invalid remote URL: ${url}. ` +
-				`Expected origin to be ${officialRemoteOrigin}/remote.html.`
+				'Expected remote URL to have a path of "/remote.html" based ' +
+				`on one of the following origins:\n ${validRemoteOrigins.join(
+					'\n'
+				)}`
 		);
 	}
 }

packages/playground/remote/remote.html

@@ -53,6 +53,7 @@
 				}
 			}
 		</style>
+		<!-- TODO: Support inserting other origin-trial codes on build, so self-hosted Playground can opt into the trials. -->
 		<!--
 		Enable the JSPI origin trial.
 		Refresh periodically at https://developer.chrome.com/origintrials/#/trials/active

packages/playground/website/public/wordpress.html

@@ -213,9 +213,12 @@
 			}
 
 			// Redirect to the Playground site with the Blueprint to download and apply the PR
+			const blueprintSchemaUrl = new URL(
+				'blueprint-schema.json',
+				location.href
+			);
 			const blueprint = {
-				$schema:
-					'https://playground.wordpress.net/blueprint-schema.json',
+				$schema: blueprintSchemaUrl.href,
 				landingPage: urlParams.get('url') || '/wp-admin',
 				login: true,
 				preferredVersions: {