Display images with new image viewer (ScrollView version)

[joshuatbrown]

Issues covered

#1164

Description

Displays full-screen images with the new image viewer. The gallery view will be covered in #1177.

How to test

1. Navigate to a post with an image
2. Tap on it
3. You're in the new image viewer!
4. Double-tap to zoom in
5. Drag around to pan
6. Double-tap again to zoom out
7. Swipe down or tap the x to dismiss

Screenshots/Video

Coming soon!

Notes

#1408 and #1413 are the same thing, but using different implementations. The point of having three draft PRs is to decide on the right approach -- each has its own pros and cons.

Pros for this approach

1. This is the simplest implementation by far.
2. It's pure SwiftUI.
3. Works as expected on Mac. You can swipe on a mouse or trackpad, or click and drag.

Cons for this approach

1. Swipe down to dismiss currently isn't implemented. It probably can be, though I haven't had any luck yet.
2. Pinching to zoom isn't implemented here. It's not a requirement right now, but if it is in the future, this approach may not work as well as the others.
3. Double-tapping to zoom will zoom in on the middle of the image, even if you double-tap the top left or bottom right corner. It doesn't zoom where you tapped.
4. On a portrait image, if you double-tap to zoom, you can scroll past the top or bottom of the image. Here's an example:

[joshuatbrown]

We're moving forward with #1413.