Fix routing loop

[chnn]

Closes #12892

If a user's session expires while they are viewing a modal, they will be taken through the following router flow:

1. Start at

   /dashboards <-- current route
   

2. Open modal, pushes modal route onto stack

   /dashboards/:dashboardID/export <--
   /dashboards
   

3. Session expires, push signin route onto stack

   /signin?returnTo=/dashboards/:dashboardID/export <--
   /dashboards/:dashboardID/export
   /dashboards
   

4. After signin, push returnTo onto stack

   /dashboards/:dashboardID/export <--
   /signin?returnTo=/dashboards/:dashboardID/export
   /dashboards/:dashboardID/export
   /dashboards
   

5. Dismiss modal, which pops last route

   /signin?returnTo=/dashboards/:dashboardID/export <--
   /dashboards/:dashboardID/export
   /dashboards
   

This creates an infinite loop 💥

Instead, we want:

1. Start at

   /dashboards <--
   

2. Open modal, pushes modal route onto stack

   /dashboards/:dashboardID/export <--
   /dashboards
   

3. Session expires, replace current route with signin route

   /signin?returnTo=/dashboards/:dashboardID/export <--
   /dashboards
   

4. After signin, replace sign in route with returnTo route

   /dashboards/:dashboardID/export <--
   /dashboards
   

5. Dismiss modal, which pops last route

   /dashboards <--