This diff factors out the last `ImageModal` feature specific part out of `FullScreenViewModal`. This logic calculates and returns the dimensions (width + height) of an image. We will need to calculate the dimensions of User Avatars in a different way, so it makes sense that this logic should now live in just `ImageModal`.
This is step 5 of the list below
Outlined below are the steps I will take in this stack (each point here will be it's own diff):
1. Introduce `FullScreenViewModal` file and move everything from the `ImageModal` file into here, and change what is absolutely necessary (the imports)
2. Correct the naming inside of `FullScreenViewModal`
3. Lift out rendering of `<Multimedia />` in favor for a generic content child
4. Lift save content / copy content functionality out of `FullScreenViewModal` and make those optional props
5. Lift the `imageDimensions` function out of `FullScreenViewModal` and introduce a required generic `contentDimensions` prop
Depends on D9362