Tapping on an uploaded picture in the post editor should bring up a larger preview

**Is your feature request related to a problem? Please describe.**
After uploading a picture in the post editor, Voyager only shows a tiny preview. Understandable, because a larger version might obscure the inputs below, but also annoying if you're struggling with writing a good title and want to avoid writing the same text that's already in the meme.


**Describe the solution you'd like**
When tapping the preview image in the post editor, a lightbox view should pop up showing a full size preview. Basically like the one that's available when viewing a regular post, sans the voting and share buttons at the bottom.

**Describe alternatives you've considered**
Open the photos app, find the picture there, and then switch back to Voyager, I guess. Doable but annoying, especially if you have a LOT of pictures.

**Additional context**
I tried implementing this myself by using the existing `GalleryImg` component in `PhotoPreview`, but even after moving the `GalleryProvider` outside of the `PageContext` I can't get it to work. The lightbox always comes up empty and worse yet, the close button doesn't even work. Not sure what I'm missing, but with the right clue I might be able to finish this myself.

Best I can currently do is probably just removing the size restriction so tapping on the preview would show a larger version *inside* the post editor, not in a lightbox.