[web] [lib] Introduce getMessagePreview function

Summary:
Moved logic for displaying message preview data into common function, to reuse it in modal with subchannels list.
No logic changes has been made.

Test Plan: In web app, message previews in chat list should display exactly as before.

Reviewers: palys-swm, benschac, atul

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3597