[web] Use <a> instead of <div> in some places

Summary:
Part of ENG-843
According to the linked article that @tomek linked in the comments of the task, we should use <a> in some places (where it brings user to a new chat/ screen).

Because of these changes we no longer use the Button for the settings button, so the round variant is no longer used and could be removed.

Test Plan: Check if all changed buttons look the same. Check if the changed css classes are used anywhere else. Check if rest of the app looks the same.

Reviewers: tomek, atul, abosh

Reviewed By: tomek, atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5327