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/Preferences.jsx

@@ -59,7 +59,7 @@ const maybeUpdateAccountSettings = async (payload) => {
   } catch (e) {
     console.log(`[Preferences] Error updating account settings`, e);
     if (e instanceof UnauthorizedError) {
-      session.resetAndRedirect(routes.login);
+      await session.resetAndRedirect(routes.login);
     }
   }
 };