Display overlay screen for blocking operations (#186)

* Catch state error in homepage widget

* Add flutter_overlay_loader lib

- Displays an overlay screen to indicate blocking operation

* Wrap blocking widget transitions in a loading overlay

- Prevents user from doing other things while loading
- Shows the user that something is happening

* Linting fixes

* Show overlay when uploading attachment file

* Show overlay when downloading file also

* Show overlay when loading or submitting API forms

- Major improvements to usability "feel"

* UI improvements for stock item test results widget

* Fix API_FORM bug

- onSuccess function was not being called

lib/widget/progress.dart

@@ -1,6 +1,7 @@
 
 
 import "package:flutter/material.dart";
+import "package:flutter_overlay_loader/flutter_overlay_loader.dart";
 
 /*
  * Construct a circular progress indicator
@@ -10,4 +11,17 @@ Widget progressIndicator() {
   return Center(
     child: CircularProgressIndicator()
   );
-}
+}
+
+
+void showLoadingOverlay(BuildContext context) {
+  Loader.show(
+    context,
+    themeData: Theme.of(context).copyWith(colorScheme: ColorScheme.fromSwatch())
+  );
+}
+
+
+void hideLoadingOverlay() {
+  Loader.hide();
+}