Fix session replica behaviour (merge with session)

The harder-to-refactor parts are the places where exists/username/token
are called within a React component. However, `resetAndRedirect` and
`store` are already called from async contexts, so adding an `await`
is simple.

This thus merges the logic, keeping localStorage for the components to
call, but making sure reset/store behaviour works correctly for the
replica.

web/src/components/UpgradeDialog.jsx

@@ -140,7 +140,7 @@ const UpgradeDialog = (props) => {
     } catch (e) {
       console.log(`[UpgradeDialog] Error changing billing subscription`, e);
       if (e instanceof UnauthorizedError) {
-        session.resetAndRedirect(routes.login);
+        await session.resetAndRedirect(routes.login);
       } else {
         setError(e.message);
       }