Screen size becomes null and IIIF images are requested at maximum size when after a store-wide reset

The display module hosts a core part of IIIF image display:

screen: {
  width: 1024,
  height: 768
}

This single state is then used by two getters iiifWidth and iiifHeight providing the maximum sizes the frontend is supposed to request to lower bandwidth consumption. To set the values in screen, vue/App.vue calls setScreenSize when the frontend loads, but it is no longer called after a store-wide reset triggered by a login or logout. Therefore, logging in or out causes images to be requested at maximum size (because the iiifUri helper in js/store/helpers.js ignores zeros or nulls) and causes unexpected image size warnings (requests#18).