[lib] put messagePreview function for mutlimedia and reaction message into respecitve message spec

Summary:
Both multimedia and reaction messages were calling the messagePreview function. To keep the message specs more modular, we should move messagePreview into individual message specs. This was suggested by Ashoat in D5689

Linear Task: ENG-2393

Test Plan:
Please watch the demo video to see that everything works as expected and there are no regressions:

Reviewers: atul, rohan, tomek

Reviewed By: atul, tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5886