[web] Use plain Button instead of html elements

Summary:
Part of ENG-843
Uses plain variant of Button introduced in a previous diff for most of the html <button>s and <div>s.

Test Plan: Check if all changed buttons look and work the same. Check if the changed css classes are used in other places.

Reviewers: tomek, atul, abosh, ginsu, O2 Blocking Reviewers

Reviewed By: atul, ginsu, O2 Blocking Reviewers

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5343