Show almost-instant preview when opening lightbox (#6000)

* Plumb thumbUri down to the lightbox

* Remove onLoad tracking from lightbox

* Hook up placeholder URI to the image

* Fix NaN causing crash on double tap while offline

* Protect against NaNs in the future

src/state/lightbox.tsx

@@ -10,6 +10,7 @@ type ProfileImageLightbox = {
 
 type ImagesLightboxItem = {
   uri: string
+  thumbUri: string
   alt?: string
 }