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/services/network.service.dart

@@ -76,9 +76,10 @@ class NetworkService {
       return res;
     } on DioError catch (e) {
       debugPrint("DioError: ${e.response}");
-      return false;
+      return null;
     } catch (e) {
       debugPrint("ERROR BackupService: $e");
+      return null;
     }
   }