Reset the app on requester change rather than performing another lookup

When changing the requester, there was no visual feedback that the app was looking up the user's information. Instead, it would show the stale data for some time and then switch. 

This change means the loader will be shown immediately.

Note that there's an existing bug with `this.ticket().requester().id()` which will mean, even after seeing the visual feedback, the same requester will always be shown. This does **not** solve that issue: https://support.zendesk.com/agent/#/tickets/382764

/cc @maximeprades