118 - Implement shared album feature (#124)

* New features 
  - Share album. Users can now create albums to share with existing people on the network.
  - Owner can delete the album.
  - Owner can invite the additional users to the album.
  - Shared users and the owner can add additional assets to the album.
* In the asset viewer, the user can swipe up to see detailed information and swip down to dismiss.
* Several UI enhancements.

mobile/lib/shared/views/immich_loading_overlay.dart

@@ -0,0 +1,41 @@
+import 'package:flutter/material.dart';
+import 'package:immich_mobile/shared/ui/immich_loading_indicator.dart';
+
+class ImmichLoadingOverlay extends StatelessWidget {
+  const ImmichLoadingOverlay({
+    Key? key,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return ValueListenableBuilder<bool>(
+      valueListenable: ImmichLoadingOverlayController.appLoader.loaderShowingNotifier,
+      builder: (context, shouldShow, child) {
+        if (shouldShow) {
+          return const Scaffold(
+            backgroundColor: Colors.black54,
+            body: Center(
+              child: ImmichLoadingIndicator(),
+            ),
+          );
+        } else {
+          return Container();
+        }
+      },
+    );
+  }
+}
+
+class ImmichLoadingOverlayController {
+  static final ImmichLoadingOverlayController appLoader = ImmichLoadingOverlayController();
+  ValueNotifier<bool> loaderShowingNotifier = ValueNotifier(false);
+  ValueNotifier<String> loaderTextNotifier = ValueNotifier('error message');
+
+  void show() {
+    loaderShowingNotifier.value = true;
+  }
+
+  void hide() {
+    loaderShowingNotifier.value = false;
+  }
+}