In this diff, I factored out mediaModalItem from FullScreenViewModal and moved that logic into MultimediaModal. This diff also factors out the necessary styles, and the dimensions state which is used to help render mediaModalItem.

This is step 2 in 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. Lift the rendering of mediaModalItem out of FullScreenViewModal
3. Finish renaming + updating remaining variables/callbacks to make FullScreenViewModal completly generic
4. Introduce UserProfileAvatarModal

Part of this linear task: https://linear.app/comm/issue/ENG-5201/factor-out-viewing-image-multimedia-messages-specific-code-from

Depends on D9495

Confirmed that there were no regressions with viewing a multimedia message in a full screen view