[web] add ellipsis icon to the left of the "see more..." text

Summary:
add ellipsis icon to the left of the "see more..." text in the ChatThreadListSeeMoreSidebars component using the ionicons icon pack

Depends on D5108

Test Plan:
Please view the screenshots to see the before and after of the changes I made

Before:

After:

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5141