[lib,keyserver] 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 keyserver and lib

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

Reviewers: palys-swm, ashoat, def-au1t

Reviewed By: ashoat

Subscribers: Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4536