Main screen loading indicator (#183)

* Bug fix for login screen

- Prevent setState() from being called if the widget is no longer loaded

* Add callback function when API status changes

- Home screen uses this function to update connection status indicator

* Linting fixes

assets/release_notes.md

@@ -9,6 +9,7 @@
 - Indicate available quantity in stock detail view
 - Adds configurable filtering to various list views
 - Allow stock location to be "scanned" into another location using barcode
+- Improves server connection status indicator on home screen
 
 ### 0.7.3 - June 2022
 ---