[web] Rename thread to chat

Summary:
Linear issue:
https://linear.app/comm/issue/ENG-1357/rename-sidebar-to-thread-in-product-copy
Diff renames thread to chat in all strings displayed to the user in web

Test Plan: Checked if strings changed as expected in web.

Reviewers: palys-swm, ashoat, def-au1t, yayabosh

Reviewed By: ashoat, yayabosh

Subscribers: Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4539