Attempt fix progress indicator

[Flajt]

-Fix: Move BlocListener to page and not button

• Fix: Use compute to reduce stress on main isolate which allows propper display of loading page indicator
• Fix: Change pushNamed to pushReplacement, there was a bug where multiple instances of the same page where on the stack and not popped, resulting in stacking calls to the PreperationBloc, which in turn leads to performance and interface issues (if I understood it right)