[web] Add unread indicator to sidebar modals
Summary: This is the second part of ENG-2089. This diff adds the basic unread indicators to the sidebar modal. Changes font color and font weight of the last unread message.
Test Plan:
Screenshots that show that unread thread messages appear unread in the sidebar modal in both 'All Threads' and 'My Threads' tabs.
| Unread thread message | Read thread message |
|---|---|
 |  |
  |   |
Reviewers: atul, ginsu, rohan
Reviewed By: atul
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D5833