(core) support ?embed=true and &style=light for a clean embed experience

Summary:
This adds query parameters useful for tailoring the Grist experience, with an eye to embedding.

Setting `style=light` removes side and top bars, as a first pass at a focused view of a single document page (this would benefit from refining).

Setting `embed=true` has no significant effect just yet other than it restricts document access to viewer at most (this can be overridden by specifying `/m/default`).

Test Plan: added tests

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2585

app/client/components/UnsavedChanges.ts

@@ -0,0 +1,48 @@
+/**
+ * Module to help deal with unsaved changes when closing a page.
+ */
+import {Disposable} from 'grainjs';
+
+/**
+ * Create an UnsavedChanges object to indicate there are UnsavedChanges. Dispose it when this is
+ * no longer the case. The optional callback will be called to confirm there are indeed unsaved
+ * changes. If omitted, it is assumed that there are.
+ */
+export class UnsavedChange extends Disposable {
+  constructor(
+    // If given, saveChanges() will call it to save changes.
+    private _saveCB?: () => Promise<void>,
+    // If given, it may return false to indicate that actually nothing has changed.
+    private _haveChanges?: () => boolean,
+  ) {
+    super();
+    unsavedChanges.add(this);
+    this.onDispose(() => unsavedChanges.delete(this));
+  }
+  public haveUnsavedChanges() { return !this._haveChanges || this._haveChanges(); }
+  public async save(): Promise<void> { return this._saveCB && this._saveCB(); }
+}
+
+export class UnsavedChangeSet {
+  private _changes = new Set<UnsavedChange>();
+
+  /**
+   * Check if there are any unsaved changes out there.
+   */
+  public haveUnsavedChanges(): boolean {
+    return Array.from(this._changes).some((c) => c.haveUnsavedChanges());
+  }
+
+  /**
+   * Save any unsaved changes out there.
+   */
+  public async saveChanges(): Promise<void> {
+    await Promise.all(Array.from(this._changes).map((c) => c.save()));
+  }
+
+  public add(unsaved: UnsavedChange) { this._changes.add(unsaved); }
+  public delete(unsaved: UnsavedChange) { this._changes.delete(unsaved); }
+}
+
+// Global set of UnsavedChanges, checked on page unload.
+export const unsavedChanges = new UnsavedChangeSet();