[web] make FullScreenViewModal completly generic

Summary:
This is a simple rename diff to make FullScreenViewModal completely generic

This is step 3 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 D9496

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

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9497